Water-absorbent resin and production process therefor

ABSTRACT

In a production process for a water-absorbent resin, comprising the steps of: blending a liquid material and a water-absorbent resin; and heating the resultant mixture in order to produce a modified water-absorbent resin, the present invention is to provide: a method for uniformly and efficiently treating a water-absorbent resin favorably in view of industry, and as a result, a good-balanced water-absorbent resin having various excellent properties, such absorption capacity without a load, absorption capacity under a load, and single-layer absorption capacity under a load in contact with an aqueous liquid. The production process comprises the step of spray-blending a water-absorbent resin (A) and a liquid material (B) with a blending apparatus equipped with a spray nozzle (C), wherein the liquid material (B) is sprayed from the spray nozzle (C) and its spray pattern is a circular and hollow cone shape or a double-convex-lens and elliptic cone shape. In addition, the production process comprises the step of heat-treating a water-absorbent resin under an atmosphere having a dew point of not higher than 60° C. and a temperature of not lower than 90° C., wherein the water-absorbent resin is obtained after a drying step following a pulverization step.

BACKGROUND OF THE INVENTION

A. Technical Field

The present invention relates to a water-absorbent resin and aproduction process therefor. More particularly, the present inventionrelates to a production process for a modified water-absorbent resin bycarrying out a specific step, and a novel water-absorbent resin assurface-crosslinked with a polyhydric alcohol.

B. Background Art

In recent years, water-absorbent resins are widely used as one ofmaterials constituted of sanitary materials, such as disposable diapers,sanitary napkins, and incontinent pads, for the purpose of causing thewater-absorbent resins absorb much water. In addition to the sanitarymaterials, the water-absorbent resins are widely used as dripping sheetsfor soil water-holding agents and foods, for the purpose of absorbing orretaining water.

As to these water-absorbent resins, the following are known as theirexamples: hydrolyzed copolymers of starch-acrylonitrile(JP-B-433951/1974), neutralized graft polymers of starch-acrylic acid(JP-A-125468/1976), saponified copolymers of vinyl acetate-acrylic acidester (JP-A-14689/1977), and hydrolyzed copolymers of acrylonitrile oracrylamide (JP-B-15959/1978), or crosslinked polymers of thesehydrolyzed copolymers, and partially-neutralized crosslinkedpoly(acrylic acid) (JP-A-84304/1980).

These water-absorbent resins are generally obtained by polymerizing anddrying, and pulverizing and classifying when the occasion demands.However, the above water-absorbent resins are usually modified byfurther adding various compounds to the resultant water-absorbent resinsso that the water-absorbent resins would have additional function afterpolymerizing and drying.

It is said that the above-mentioned water-absorbent resins should beexcellent in the following properties: the absorption capacity, theabsorption speed, the liquid permeability, the gel strength of hydrogel,the suction power to suck up water from a base material containing anaqueous liquid, and so on, upon contact with an aqueous liquid such as abody fluid. However, relations between these properties do notnecessarily display positive correlations. For example, as theabsorption capacity increases, some other properties such as liquidpermeability, gel strength, and absorption speed deteriorate. Inaddition, there are some water-absorbent resins having higher absorptioncapacity, which form so-called fish eyes in contact with an aqueousliquid, and have extremely low absorption capacity under a load becausewater is not dispersed in the entirety of the water-absorbent resinparticles.

When producing the water-absorbent resins, as to a method for modifyingthe above-mentioned water-absorption properties of the water-absorbentresin in good balance, namely, as to a method for modifying thewater-absorbent resin, an art in which the neighborhood of the surfaceof the water-absorbent resin is crosslinked, what is called, asurface-crosslinking art is known. Examples of these crosslinking agentsas used are polyhydric alcohols, polyglycidyl ethers, haloepoxycompounds, polyaldehydes, polyamines, and polyvalent metal salts.

The most important matter thought in this surface-crosslinking step isto surface-crosslink the surface of water-absorbent resin particlesuniformly, and therefore, it is important that the water-absorbent resinbefore surface-crosslinking is uniformly blended with asurface-crosslinking agent. As to an art in which this water-absorbentresin before surface-crosslinking is uniformly blended with thesurface-crosslinking agent, various methods are disclosed until now. Forexample, the following methods are known: a method which involves theuse of crosslinking agents having a different solubility parametertogether (JP-A-184320/1994 (corresponding to U.S. Pat. No. 5,422,405));a method which involves the use of a specific material as the materialof the inner surface of the blender, and involves add-blending anaqueous crosslinking agent liquid while being stirred in a high speed(JP-A-235378/1997 (corresponding to U.S. Pat. No. 6,071,976) andJP-A-349625/1999); and a method involves spraying a particulate liquiddrop of a surface-crosslinking agent to bring a water-absorbent resinpowder in a row state (JP-A-246403/1992).

As to methods for surface-crosslinking water-absorbent resins usingthese crosslinking agents, the following methods are known: a methodwhich involves directly adding a crosslinking agent to a water-absorbentresin powder, or a composition obtained by dissolving a crosslinkingagent in a small quantity of water or a hydrophilic organic solvent, andheat-treating if necessary (JP-A-180233/1983 (corresponding to U.S. Pat.No. 4,666,983), JP-A-189103/1984, and JP-A-16903/1986 (corresponding toU.S. Pat. No. 4,734,478)); a method which involves dispersing awater-absorbent resin in a mixed solvent of water and a hydrophilicorganic solvent, and adding a crosslinking agent thereto to react withthe water-absorbent resin (JP-B-48521/1986); and a method which involvesallowing a resin to react with a crosslinking agent in an inert solventin the presence of water (JP-B-18690/1985 (corresponding to U.S. Pat.No. 45,418,771)).

Then, when a water-absorbent resin is surface-crosslinked, the moderatepermeation of a crosslinking agent to the neighborhood of awater-absorbent resin powder is an important factor, and it is necessarythat its process is favorable in view of industry.

In addition, the state of the water-absorbent resin is a powder in manycases. When the water-absorbent resin includes many particulate powderssuch that pass through a sieve having a mesh opening size of 150 μm, itmay exercise a bad influence on the working environment due to causingdust, it may cause the blendability to decrease when blending with othersubstances, and it may cause formation of bridge in a hopper.

Until now, known examples of production processes for water-absorbentresins having a small amount of particulate powders include a methodwhich involves adjusting the particle diameter by adjusting the extentof the polymerization or pulverization, or a method which involvesclassify-removing the particulate powders as caused. However, plenty ofparticulate powders (several to several tens percents) are caused in theproduction steps even if the above method is carried out. Therefore, theyield is greatly decreased when the particulate powders areclassify-removed, and further abandoned. At the same time, there aredisadvantages in view of abandoning cost.

Then, various methods for modifying a water-absorbent resin for thepurpose of solving the above problems are proposed by granulating orrecovering the particulate powders to granules by use of binders such asan aqueous liquid, wherein the particulate powders are inevitably causedin the steps of producing the water-absorbent resin (JP-A-101536/1986(corresponding to U.S. Pat. No. 4,734,478) and JP-A-817200/1991(corresponding to U.S. Pat. No. 5,369,148)). Preferred binders for thewater-absorbent resin generally include water or an aqueous liquid inview of efficiency, safety, and production costs.

The steps of producing such a water-absorbent resin includes anmodifying step by adding and blending a liquid material, such as addinga surface-crosslinking agent to the water-absorbent resin afterpolymerization and drying, or blending a binder to a water-absorbentresin including powders in order to reduce dust as caused. In addition,when carrying out an antimicrobial processing, an removal of odor, andbesides, an modification of giving additional functions to awater-absorbent resin, which tend to increase in recent years, thewater-absorbent resin is frequently blended with antimicrobial agents,deodorants, and besides, other additives, as a liquid material.

Furthermore, when the water-absorbent resin is surface-crosslinked ormodified, a liquid material is added (preferably spray-added), and thenthe resultant mixture is heat-treated. However, even if thewater-absorbent resin is heated at the same temperature (water-absorbentresin temperature or heat medium temperature), depending upon the kindof liquid materials, the improvement of the properties might beinsufficient, and the properties might not be stabilized in a continuousproduction process.

When producing the water-absorbent resin, the step of blending theliquid material to the water-absorbent resin is essential so as tomodify its various properties with good balance, and further, to giveadditional functions. However, the water-absorbent resin has a propertyof absorbing the liquid material rapidly when the water-absorbent resincomes into contact with the liquid material. Therefore, it is difficultto blend the liquid material with the water-absorbent resin uniformly.

In addition, the water-absorbent resin has a characteristic ofincreasing adhesion when the water-absorbent resin absorbs a liquid.Therefore, the water-absorbent resin excessively absorbing the liquidmay be formed as an adhesive or piled material in a blending apparatus.When operating the blending apparatus in order to mass-produce thewater-absorbent resin, the formation of such a piled material causes anoverload for a driving motor of such as a driving shaft, and is aserious problem on operating the apparatus safely.

In addition, the surface-treatment, such as forming surface-crosslinkinglayers in a water-absorbent resin is tried so as to modify variousproperties of the water-absorbent resin in good balance in the aboveway. However, any treatment has above-mentioned problems, and there wasno sufficiently satisfactory method in view of property and industrybefore.

SUMMARY OF THE INVENTION

A. Object of the Invention

Accordingly, in a production process for a water-absorbent resin,comprising the steps of: blending a liquid material and awater-absorbent resin; and heating the resultant mixture in order toproduce a modified water-absorbent resin, an object of the presentinvention is to provide: a method for uniformly and efficiently treatinga water-absorbent resin favorably in view of industry, and as a result,a good-balanced water-absorbent resin having various excellentproperties, such absorption capacity, absorption capacity under a load,and single-layer absorption capacity under a load in contact with anaqueous liquid.

B. Disclosure of the Invention

The present inventors diligently studied to solve the problems. As aresult, they found that the problems could be solved by employing a modeof spray-blending with a specific blending apparatus and/or a mode ofspecific surface-treatment in a surface-treating step.

That is to say, a production process for a water-absorbent resin,according to the present invention, comprises the steps of: blending aliquid material and a water-absorbent resin; and heating the resultantmixture in order to produce a modified water-absorbent resin, and ischaracterized by further comprising the step of spray-blending awater-absorbent resin (A) and a liquid material (B) with a blendingapparatus equipped with a spray nozzle (C), and being characterized inthat the liquid material (B) is sprayed from the spray nozzle (C) andits spray pattern is a circular and hollow cone shape.

In addition, another production process for a water-absorbent resin,according to the present invention, comprises the steps of: blending aliquid material and a water-absorbent resin; and heating the resultantmixture in order to produce a modified water-absorbent resin, and ischaracterized by further comprising the step of spray-blending awater-absorbent resin (A) and a liquid material (B) with a blendingapparatus equipped with a spray nozzle (C), and being characterized inthat the liquid material (B) is sprayed from the spray nozzle (C) andits spray pattern is a double-convex-lens and elliptic cone shape.

In addition, yet another production process for a water-absorbent resin,according to the present invention, comprises the steps of: blending aliquid material (B) and a water-absorbent resin (A); and heating theresultant mixture in order to produce a modified water-absorbent resin,and is characterized by further comprising the step of heat-treating awater-absorbent resin under an atmosphere having a dew point of nothigher than 60° C. and a temperature of not lower than 90° C., whereinthe water-absorbent resin before modifying is obtained after a dryingstep following a pulverization step.

In addition, yet another production process for a water-absorbent resin,according to the present invention, comprises the steps of: blending aliquid material and a water-absorbent resin; and heating the resultantmixture in order to produce a modified water-absorbent resin, andfurther comprises the steps of: spray-blending a water-absorbent resin(A) and a liquid material (B) with a blending apparatus equipped with aspray nozzle (C); and heat-treating, with the production process beingcharacterized in that the liquid material (B) is sprayed from the spraynozzle (C) and its spray pattern is a circular and hollow cone shape inthe spray-blending step, and in that the heat-treating step is carriedout under an atmosphere having a dew point of not higher than 60° C. anda temperature of not lower than 90° C.

In addition, yet another production process for a water-absorbent resin,according to the present invention, comprises the steps of: blending aliquid material and a water-absorbent resin; and heating the resultantmixture in order to produce a modified water-absorbent resin, andfurther comprises the steps of: spray-blending a water-absorbent resin(A) and a liquid material (B) with a blending apparatus equipped with aspray nozzle (C); and heat-treating, with the production process beingcharacterized in that the liquid material (B) is sprayed from the spraynozzle (C) and its spray pattern is a double-convex-lens and ellipticcone shape in the spray-blending step, and in that the heat-treatingstep is carried out under an atmosphere having a dew point of not higherthan 60° C. and a temperature of not lower than 90° C.

In addition, a water-absorbent resin, according to the presentinvention, is surface-crosslinked with a surface-crosslinking agentincluding at least a polyhydric alcohol, has a particle sizedistribution such that the ratio of particles having particle diametersof smaller than 150 μm is not more than 5 weight %, and exhibits anabsorption capacity without a load of not less than 30 g/g, with thewater-absorbent resin being characterized in that: the single-layerabsorption capacity (10 min.) of particles having particle diameters of600 to 300 μm is not less than 30 g/g under a load; the single-layerabsorption capacity (60 min.) of particles having particle diameters of600 to 300 μm is not less than 30 g/g under a load; the single-layerabsorption capacity (10 min.) of particles having particle diameters of300 to 150 μm is not less than 30 g/g under a load; and the single-layerabsorption capacity (60 min.) of particles having particle diameters of300 to 150 μm is not less than 30 g/g under a load.

In addition, a water-absorbent resin, according to the presentinvention, is surface-crosslinked with a surface-crosslinking agentincluding at least a polyhydric alcohol, has a particle sizedistribution such that the ratio of particles having particle diametersof smaller than 150 μm is not more than 5 weight %, and exhibits anabsorption capacity without a load of not less than 30 g/g, with thewater-absorbent resin being characterized in that the index of uniformsurface-treatment is not less than 0.70.

In addition, a sanitary material, according to the present invention,comprises the water-absorbent resin according to the present invention.

These and other objects and the advantages of the present invention willbe more fully apparent from the following detailed disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the spray pattern of the circular andhollow cone shape (a hollow cone spray shape), which is used forproducing water-absorbent resins according to the present invention.

FIG. 2 is a schematic view of the spray pattern of thedouble-convex-lens and elliptic cone shape (a flat spray shape), whichis used for producing water-absorbent resins according to the presentinvention.

FIG. 3 is a schematic view of a measurement apparatus for measuring thewater absorption capacity under a load, which is one of properties ofwater-absorbent resins in the present invention.

FIG. 4 is a figure showing the relationship between relational humidityand temperature (° C.) as to dew curves of vapor. In addition, the scopelimited in the present invention claim is described in the hatchingrange. Td, black plots, and white plots mean a dew temperature, examplesof the present invention, and comparative examples, respectively.

EXPLANATION OF THE SYMBOLS

1: Balance

2: Vessel

3: Air-inhaling pipe

4: Introducing tube

5: Measurement part

6: Glass filter

7: Filter paper

8: Supporting cylinder

9: Wire net

10: Weight

11: Physiological saline solution or synthetic urine

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention is explained in detail.

(Water-absorbent resin before modifying):

The water-absorbent resin added to the liquid material is not especiallylimited in the present invention, and can fitly be determined accordingto its use. However, a hydrophilic crosslinking polymer having acarboxyl group is preferably used. The hydrophilic crosslinking polymeris a conventional water-absorbent resin which is obtained bypolymerizing hydrophilic monomers comprising a major proportion ofeither or both of acrylic acid and its salt (neutralized product), andforms a hydrogel in water due to the absorption of as large an amount ofwater as 50 to 1,000 times of themselves.

The acid group of the hydrophilic crosslinking polymer is morepreferably neutralized with alkali metal salts, ammonium salts, or aminesalts, for example, preferably in a ratio of 30 to 100 mol %, morepreferably 50 to 90 mol %, particularly preferably 60 to 80 mol %. Thepolymerization reaction may be started after the beforehand neutralizingthis acid group in a step of preparing a hydrophilic unsaturated monomerbefore obtaining the hydrophilic crosslinking polymer, or the acid groupof the hydrophilic crosslinking polymer as obtained during or after thepolymerization reaction may be neutralized, or these may be combinedeach other.

The hydrophilic unsaturated monomer may include unsaturated monomersother than acrylic acid or its salt (hereinafter, referred as othermonomers) when the occasion demands. Example of the other monomerinclude: anionic unsaturated monomers, such as methacrylic acid, maleicacid, vinylsulfonic acid, styrenesulfonic acid,2-(meth)acrylamido-2-methylpropanesulfonic acid,2-(meth)acryloylethanesulfonic acid, and 2-(meth)acryloylpropanesulfonicacid, and their salts; nonionic unsaturated monomers containinghydrophilic groups, such as acrylamide, methacrylamide,N-ethyl(meth)acrylamide, N-n-propyl(meth)acrylamide,N-isopropyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide,2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate,methoxypolyethylene glycol (meth)acrylate, polyethylene glycolmono(meth)acrylate, vinylpyridine, N-vinylpyrrolidone,N-acryloylpiperidine, and N-acryloylpyrrolidine; and cationicunsaturated monomers such as N,N-dimethylaminoethyl (meth)acrylate,N,N-diethylaminoethyl (meth)acrylate, N,N-dimethylaminopropyl(meth)acrylate, N,N-dimethylaminopropyl (meth)acrylamide, and theirquaternary salts. However, the other monomers are not especially limitedthereto. When these other monomers are jointly used, the amount as usedis preferably not more than 30 mol %, more preferably not more than 10mol %, of the entirety of the hydrophilic unsaturated monomer.

The water-absorbent resin as obtained by polymerizing the hydrophilicunsaturated monomer preferably has carboxyl groups. The amount of thecarboxyl group in the water-absorbent resin is not especially limited,but is preferably not less than 0.01 equivalent per 100 g of thewater-absorbent resin.

When obtaining the water-absorbent resin, a crosslinked structure isdesirably introduced into the polymer by using an internal-crosslinkingagent. The above-mentioned internal-crosslinking agent may be a compoundhaving a plurality of polymerizable unsaturated groups and/or reactivegroups to the carboxyl group per molecule, and is not especiallylimited. That is to say, the internal-crosslinking agent may be acompound having a plurality of substituent groups copolymerizable withthe hydrophilic unsaturated monomer and/or reactive to the carboxylgroup of the carboxyl hydrophilic unsaturated monomer per molecule.Incidentally, the hydrophilic unsaturated monomer may comprise aself-crosslinking compound which forms the crosslinked structure even ifthe internal-crosslinking agent is not used.

Examples of the above-mentioned internal-crosslinking agent includeN,N′-methylenebis(meth)acrylamide, (poly)ethylene glycoldi(meth)acrylate, (poly)propylene glycol di(meth)acrylate,trimethylolpropane tri(meth)acrylate, trimethylolpropanedi(meth)acrylate, glycerol tri(meth)acrylate, glycerol acrylatemethacrylate, ethoxylated trimethylolpropane tri(meth)acrylate,pentaerythritol tetra(meth)acrylate, dipentaerythritolhexa(meth)acrylate, triallyl cyanurate, triallyl isocyanurate, triallylphosphate, triallylamine, poly(meth)allyloxyalkanes, (poly)ethyleneglycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol,polyethylene glycol, propylene glycol, glycerin, pentaerythritol,ethylenediamine, polyethylenimine, and glycidyl (meth)acrylate. However,the internal-crosslinking agent is not especially limited. Theseinternal-crosslinking agents are used either alone respectively or incombinations with each other. Among the exemplifiedinternal-crosslinking agents, those having a plurality of polymerizableunsaturated groups per molecule are preferably used because they cangive a water-absorbent resin of which the properties are more modified.

The amount of the internal-crosslinking agent as used is preferably inthe range of 0.005 to 3 mol %, more preferably, 0.01 to 1.5 mol %, ofthe above-mentioned monomer. In the case where the amount of theabove-mentioned internal-crosslinking agent as used is smaller than0.005 mol % or larger than 3 mol %, water-absorbent resins havingdesired properties might not be obtained.

Incidentally, when the hydrophilic unsaturated monomer is polymerized inorder to obtain the water-absorbent resin, the following materials maybe added to the reaction system: hydrophilic polymers such asstarch-cellulose, derivatives of starch-cellulose, polyvinyl alcohol,polyacrylic acid (or its salts), and crosslinked polyacrylic acid (orits salts); chain transfer agents such as hypophosphorous acid (or itssalts); chelating agents; and water-soluble or water-dispersiblesurfactants.

The method for polymerizing the hydrophilic unsaturated monomer is notespecially limited. For example, conventional methods such as aqueoussolution polymerization, reversed-phase suspension polymerization, bulkpolymerization and precipitation polymerization, are available. Amongthese polymerizations, methods in which an aqueous solution of thehydrophilic unsaturated monomer is prepared and then polymerized,namely, the aqueous solution polymerization or reversed-phase suspensionpolymerization in consideration of the easiness of the polymerizationreaction control and the performance of the resultant water-absorbentresin.

In the above polymerization method, the concentration of the aqueousmonomer component solution, namely, the ratio of the monomer componentin the aqueous solution is not especially limited, but is preferably notless than 10 weight %, more preferably in the range of 10 to 65 weight%, still more preferably 10 to 50 weight %, most preferably 15 to 40weight %. In addition, the reaction conditions such as reactiontemperature and reaction time may fitly be set for the monomer componentas used, and are not especially limited.

When polymerizing the hydrophilic unsaturated monomers, the followingcan be used: radical polymerization initiators, such as potassiumpersulfate, ammonium persulfate, sodium persulfate, t-butylhydroperoxide, hydrogen peroxide, and 2,2′-azobis(2-amidinopropane)dihydrochloride; and active energy lights, such as ultraviolet, andelectron beam. In addition, when using the oxidative radicalpolymerization initiators, they may be combined with reducing agents,such as sodium sulfite, sodium hydrogen sulfite, iron (II) sulfate, andL-ascorbic acid, thereby carrying out redox polymerization. The amountof these polymerization initiators as used is preferably in the range of0.001 to 2 mol %, more preferably 0.01 to 0.5 mol %.

The solid content of the hydrogel polymer as obtained by theabove-mentioned polymerization is adjusted by drying. The drying of thehydrogel polymer can be carried out by using conventional dryers andheating furnaces, such as thin blending dryers, rotary dryers, diskdryers, fluidized-bed dryers, air blow type dryers, and infrared dryers.Then, the drying temperature is preferably in the range of 40 to 250°C., more preferably 90 to 200° C., still more preferably 120 to 180° C.The solid content of the dry product as obtained in the above way isusually in the range of 70 to 100 weight % (water content: 30 to 0weight %), preferably 80 to 98 weight % (water content: 20 to 2 weight%), most preferably 90 to 98 weight % (water content: 10 to 2 weight %).Incidentally, the solid content is usually calculated from the amount asdecreased by drying at 180° C. for 3 hours.

The dry product as obtained in the above drying can be used as awater-absorbent resin as it is. However, the dry product can be used asa particulate water-absorbent resin having a predetermined size bypulverization and classification. Then, the particle size is not largerthan 2 mm, preferably in the range of 10 μm to 1 mm. The weight-averageparticle diameter may be different depending upon its use, but is in therange of 100 to 1,000 μm, preferably 150 to 800 μm, still morepreferably 300 to 600 μm. In addition, the ratio of the particlespassing through a sieve having a mesh opening size of 150 μm ispreferably not more than 15 weight %, more preferably not more than 10weight %, still not more than 5 weight %.

The water-absorbent resin as obtained may be in various shapes, such asspherical shapes, flake shapes, irregular pulverized shapes, fibershapes, granular shapes, stick shape, conventional round shapes, andflat shapes.

In addition, the content of an uncrosslinked polymer in thewater-absorbent resin, namely, the extractable content is preferably notmore than 30 weight %, more preferably not more than 20 weight %, stillmore preferably not more than 10 weight %.

In addition, the present invention process is favorably applied towater-absorbent resins having high absorption capacity, which weredifficult to uniformly blend with the liquid material in the past, andis applied to water-absorbent resins having a water absorption capacitywithout load of preferably not less than 30 g/g, more preferably 35 to100 g/g, still more preferably 40 to 90 g/g, particularly preferably 45to 85 g/g.

In the present invention, the water-absorbent resin as obtained in theabove way is spray-blended with the liquid material with a specificblending apparatus, and/or specifically heat-treated. Hereinafter, theseare explained one by one.

(Step of spray-blending liquid material):

In the present invention, the liquid material is added to thewater-absorbent resin as obtained in the above way from the spraynozzle, and further modified. The modification by adding the liquidmaterial in the present invention includes at least one selected fromamong the following surface-crosslinking, granulation, and addition ofadditives. Incidentally, a water-absorbent resin before adding theliquid material (B) is simply referred as the water-absorbent resin (A),and the water-absorbent resin (A) after adding the liquid material (B)is referred as a modified or surface-crosslinked water-absorbent resin.

The powder temperature of the water-absorbent resin (A) as obtained inthe above way before adding the liquid material (B) is preferablyadjusted to the range of 80 to 35° C., more preferably 70 to 35° C.,still more preferably 50 to 35° C. Thereafter, the liquid material (B)is blended therewith. In case where the temperature of thewater-absorbent resin (A) before adding the liquid material (B) ishigher, the liquid material (B) is blended ununiformly. In addition,there are disadvantages in adjusting to lower than 35° C., because ittakes much time to forcibly or stationary cool, and besides, theagglomeration of the powder as stationary cooled is observed, and theenergy loss is increased when carrying out reheating.

When the surface neighborhood of the water-absorbent resin (A) beforesurface-crosslinking is further crosslinked, the liquid material (B)includes a surface-crosslinking agent, and is spray-blended with thespray nozzle (C) having the below-mentioned specific spray pattern. Inaddition, the resultant mixture was heat-treated, and then, thewater-absorbent resin (A) is surface-crosslinked.

The surface-crosslinking agent as comprised in the liquid material (B)is not especially limited if it is a compound which has a plurality offunctional groups in one molecule reactive upon a carboxyl group of thewater-absorbent resin (A) and can form a covalent bond by thecrosslinking reaction.

Examples of the above surface-crosslinking agent include: polyhydricalcohols, such as ethylene glycol, propylene glycol, glycerol,pentaerythritol, sorbitol, diethylene glycol, triethylene glycol,tetraethylene glycol, dipropylene glycol, tripropylene glycol,1,3-butane diol, 1,4-butanediol, 1,5-pentanediol, 2,4-pentanediol,1,6-hexanediol, 2,5-hexanediol, and trimethylolpropane; polyaminecompounds, such as diethanolamine, triethanolamine, ethylenediamine,diethylenetriamine, and triethylenetetramine; polyglycidyl compounds,such as ethylene glycol diglycidyl ether, polyethylene glycol diglycidylether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether,polyglycerol polyglycidyl ether, propylene glycol diglycidyl ether, andpolypropylene glycol diglycidyl ether; 2,4-tolylene diisocyanate,ethylene carbonate (1,3-dioxolan-2-one), propylene carbonate(4-methyl-1,3-dioxolan-2-one), 4,5-dimethyl-1,3-dioxolan-2-one, (poly-,di- or mono-) 2-oxazolidinone, epichlorohydrin, epibromohydrin, diglycolsilicate, and polyaziridine compounds, such as2,2-bis(hydroxymethylbutanol)-tris[3-(1-aziridyl)propionate]. However,the surface-crosslinking agent is not limited to these compounds. Inaddition, these surface-crosslinking agents are used either alonerespectively or in combinations with each other. Among these, at leastone kind of the surface-crosslinking agents is preferably asurface-crosslinking agent selected from the group consisting ofpolyhydric alcohols, polyglycidyl compounds, 1,3-dioxolan-2-on,poly(2-oxazolidinone), bis(2-oxazolidinone), and mono(2-oxazolidinone),and is more preferably a surface-crosslinking agent including polyhydricalcohols.

The polyhydric alcohol is a safe surface-crosslinking agent with whichthe surface-crosslinking gives high properties to the water-absorbentresin, but which was difficult to uniformly blend with thewater-absorbent resin because of high viscosity and/or hydrophilicity ofthe polyhydric alcohol. However, in the present invention, an aqueoussolution including the polyhydric alcohol as the surface-crosslinkingagent can preferably be used.

The amount of the surface-crosslinking agent as used, depends on thecompounds as used as such, or on combinations thereof, but is preferablyin the range of 0.001 to 5 parts by weight, more preferably 0.005 to 2parts by weight, per 100 parts by weight of the solid content of thewater-absorbent resin (A). In case where the amount of thesurface-crosslinking agent as used is more than the above range, thereare disadvantages in that: it is not only uneconomical but also theamount of the surface-crosslinking agent is excessive to form the mostsuitable crosslinking structure in the water-absorbent resin (A). Inaddition, in case where the amount of the surface-crosslinking agent asused is less than the above range, it might be difficult to obtain asurface-crosslinked water-absorbent resin having a higher absorptioncapacity under a load.

When the water-absorbent resin (A) is blended with thesurface-crosslinking agent, water is preferably used as a solvent, andthe liquid material (B) is preferably in a form of an aqueoussurface-crosslinking agent solution. The amount of water as used dependsupon factors such as the type, or the particle diameter of thewater-absorbent resin (A), but is preferably more than 0 part by weightand not more than 20 parts by weight, more preferably in the range of0.5 to 10 parts by weight, per 100 parts by weight of the solid contentof the water-absorbent resin (A).

When the water-absorbent resin (A) is blended with thesurface-crosslinking agent, a hydrophilic organic solvent may further beused, if necessary. Examples of the hydrophilic organic solvent include:lower alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol,isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, and t-butylalcohol; ketones such as acetone; ethers such as dioxane,tetrahydrofuran, and methoxy(poly)ethylene glycol; amides such asε-caprolactam and N,N-dimethylformamide; and sulfoxides such as dimethylsulfoxide. The amount of the hydrophilic organic solvent as used dependsupon factors such as the type, or particle diameter of thewater-absorbent resin (A), but is preferably less than 20 parts byweight, more preferably in the range of 0.1 to 10 parts by weight, stillmore preferably 0 to 5 parts by weight, particularly preferably 0 to 1part by weight, per 100 parts by weight of the solid content of thewater-absorbent resin (A). However, in the present invention, theuniform blending can be accomplished without using the hydrophilicorganic solvents particularly because of excellent mixability.

In addition, the liquid temperature of the liquid material (B) ispreferably lower than the powder temperature of the water-absorbentresin (A), more preferably lower than the powder temperature of thewater-absorbent resin (A) by 10° C., still more preferably by 20° C.,most preferably by 30° C. Incidentally, the liquid material (B) issprayed from the spray nozzle (C). Therefore, its liquid temperatureshould be higher than its melting point. In addition, when the liquidtemperature of the liquid material (B) is too high, there aredisadvantages in that: the liquid absorption speed is rapid, and theuniform blending of the liquid material (B) and the water-absorbentresin (A) is inhibited.

When the water-absorbent resin (A) is blended with the aqueoussurface-crosslinking agent solution in the present invention, theaqueous surface-crosslinking agent solution, namely, the liquid material(B) spray-blended with the water-absorbent resin (A) by use of theblender equipped with the specific spray nozzle (C).

The average diameter of the liquid drop of the liquid material (B) asblended with the water-absorbent resin (A) is preferably smaller thanthe average diameter of the water-absorbent resin (A), and is morepreferably not larger than 300 μm, still more preferably not larger than250 μm. The average diameter of the liquid drop is usually in the rangeof 50 to 200 μm. In case where the average diameter is larger than 300μm, there are disadvantages in that: it is difficult to defuse ordisperse the liquid material (B) uniformly; a lump having high densityis caused; and the amount of the water-absorbent resin (A) which doesnot come into contact with the liquid material (B), namely, the aqueoussurface-crosslinking agent solution is increased in the blendingapparatus.

In the present invention, the spray angle of the liquid material (B)from the spray nozzle (C) is very important, and the maximum spray angleof the liquid material (B) from the spray nozzle (C) is preferably notless that 50°.

The production process, according to the present invention, ischaracterized in that: the liquid material (B) is sprayed from the spraynozzle (C) and its spray pattern is a circular and hollow cone shape (ahollow cone spray shape); or the liquid material (B) is sprayed from thespray nozzle (C) and its spray pattern is a double-convex-lens andelliptic cone shape (a flat spray shape). In these processes, themaximum spray angle is preferably is not less than 50°.

The schematic figures of the spray angle and the spray pattern of thespray nozzle (C) are illustrated in FIG. 1 (spraying in a circular andhollow cone shape (a hollow cone spray shape)) and FIG. 2 (spraying in adouble-convex-lens and elliptic cone shape (a flat spray shape)).However, the spray angle is changed depending upon the viscosity andtemperature of the liquid material (B) sprayed from the spray nozzle(C). In addition, when the spray nozzle (C) is a hydraulic nozzles(liquid-pressurized nozzles sprayed only by fluid pressure), the sprayangle is changed due to the amount of the liquid material (B) as sprayedand its spray pressure. When the spray nozzle (C) is a pneumatic nozzles(air- and liquid-pressurized nozzles (sprayed with compressed air)), thespray angle is changed due to the amount of the liquid material (B) assprayed, the air pressure, and the amount of air as consumed.

As is illustrated in FIG. 1, when the liquid material (B) is sprayedfrom the spray nozzle (C) in a circular and hollow cone shape (a hollowcone spray shape), and for example, the water-absorbent resin (A) istransferred toward arrowed directions (a, b, and c) in FIG. 1, theamount as sprayed at end portions a and c, and the amount as sprayed inthe center portion b vary little while being transferred. As a result,the uniform spraying can be realized.

As is illustrated in FIG. 2, when the liquid material (B) is sprayedfrom the spray nozzle (C) in a double-convex-lens and elliptic coneshape (a flat spray shape), and for example, the water-absorbent resin(A) is transferred toward arrowed directions (a, b, and c) in FIG. 2,the amount as sprayed at end portions a and c, and the amount as sprayedin the center portion b vary little while being transferred. As aresult, the uniform spraying can be realized.

The spray nozzle (C) is necessary to be fitly selected according to theuse condition so that the spray nozzle (C) has a predetermined sprayangle. However, the spray angle of the liquid material (B) from thespray nozzle (C) is preferably selected at not less than 50°, morepreferably not less than 70°, still more preferably not less than 90°.In case where the spray angle is less than 50°, the portion where theliquid material (B) is dispersed excessively and the portion where theliquid material (B) is dispersed in low density are caused in adispersing state of the liquid material (B) as sprayed in the blendingapparatus, and the partiality is caused in a blending state of thewater-absorbent resin (A) and the liquid material (B). There aredisadvantages in that the water-absorbent resin (A) as excessivelybrought into contact with the liquid material (B), namely, the aqueoussurface-crosslinking agent solution produces a lump having high density(rigid agglomerated material) easily, and causes excessivesurface-crosslinking. This lump having high density becomes a rigid lumpdifficult to pulverize after the heat treatment as described in thefollowing. Therefore, the extra-pulverization is necessary in order toadjust to the particle size of the produced material (for example, allthe particles have a particle size of less than 1 mm.). However, wherethe pulverization is carried out, there are disadvantages in that thesurface-crosslinked layer as specially formed is destroyed bypulverization. Incidentally, the maximum spray angle is not more than180° because of the structure of the spray.

In addition, there are disadvantages in that the water-absorbent resin(A) as excessively brought into contact with the liquid material (B) iseasily adhered and piled in the blending apparatus in view of the stableoperation of the apparatus. Furthermore, when the portion where theliquid material (B) is excessively dispersed comes into contact with theportion of the blending apparatus, there are disadvantages in that theliquid drop is easily produced and the formation of the piled materialis caused in the apparatus.

On the other hand, there are disadvantages in that the desirablecrosslinking effect is not possibly obtained in the water-absorbentresin (A) as brought into contact with the liquid material (B) in lowdensity.

In addition, the present invention process which involves the maximumspray angle of the liquid material (B) from the spray nozzle (C) at notless than 50°, the process which involves spraying the liquid material(B) from the spray nozzle (C) with a spray pattern of a circular andhollow cone shape (a hollow cone spray shape), or the process whichinvolves spraying the liquid material (B) from the spray nozzle (C) witha spray pattern of a double-convex-lens and elliptic cone shape (a flatspray shape) is more preferably applied to continuous productionprocesses. Incidentally, the maximum spray angle is not more than 180°because of the structure of the spray.

Furthermore, when the liquid material (B) is sprayed from the spraynozzle (C) for the purpose of adjusting to the above predetermined sprayangle, the area of the spray-dispersing state of the liquid material (B)projected onto a sectional area which is perpendicular to the axisdirection of the blending apparatus and includes a spraying point of thespray nozzle (C) preferably accounts for 70 to 100% of the sectionalarea perpendicular to the axis direction of the blending apparatus, morepreferably 80 to 100%, still more preferably 90 to 100%. In case wherethe area of the spray-dispersing state of the liquid material (B)projected onto a sectional area which is perpendicular to the axisdirection of the blending apparatus and includes the spraying point ofthe spray nozzle (C) preferably accounts for less than 70% of thesectional area perpendicular to the axis direction of the blendingapparatus, there are disadvantages in that the portion where the liquidmaterial (B) is dispersed excessively and the portion where the liquidmaterial (B) is dispersed in low density are caused in a dispersingstate of the liquid material (B) as sprayed in the blending apparatus,and the partiality is caused in a blending state of the water-absorbentresin (A) and the liquid material (B).

The number of the spray nozzles (C) with which the blending apparatus isequipped may be one or more. The number is preferably two or more sothat the area of the spray-dispersing state projected onto a sectionalarea including the spraying point of the spray nozzle (C) would enlarge.

The blending apparatus as used for blending the water-absorbent resin(A) and the liquid material (B) is desired to have strong blendingpower, and the water-absorbent resin is preferably stirred or fluidizedby flowing gas, so that both would be uniformly and surely blended.Examples of the above blending apparatus include: cylinder typeblenders, double-walls cone type blenders, V-character-shaped blenders,ribbon type blenders, screw type blenders, fluid type rotary diskblenders, air current type blenders, double-arm type kneaders, internalblenders, pulverization type blenders, rotary blenders, and screw typeextruders. However, a high-speed agitation type blending apparatuscomprising an agitation shaft having a plurality of paddles ispreferable. Herein, the high-speed agitation type blending apparatusmeans a blender which obtains blending power by rotating the agitationshaft having a plurality of paddles usually with 100 to 5,000 rpm,preferably 200 to 4,000 rpm, more preferably 500 to 3,000 rpm.

In addition, the inner wall of the blending apparatus is preferably madeof a low adhesive material such as Teflon in order to prevent themixture of the water-absorbent resin (A) and the liquid material (B)from adhering and piling.

Furthermore, the inner wall temperature of the blending apparatus ispreferably higher than room temperature, more preferably not lower than40° C. The temperature is preferably maintained in the range of 50 to100° C. In addition, the inner wall temperature of the blendingapparatus is preferably higher than the temperature of water-absorbentresin (A). The temperature difference is preferably not more than 40°C., more preferably not more than 20° C. In case where the inner walltemperature of the blending apparatus is not higher than roomtemperature, when the liquid material (B) and water-absorbent resin (A)are blended, there is a possibility that the resultant water-absorbentresin mixture is adhered to the inner wall or piled.

The modification carried out by adding the liquid material (B),according to the present invention, can be widely applied typically toaddition of the surface-crosslinking agent in the surface-crosslinkingof the water-absorbent resin. However, there is no especial limitationthereto. For example, the modification can be applied to granulation ofwater-absorbent resins and mixing thereof with additives besides theaddition of the surface-crosslinking agent. The water-absorbent resin(A) which would be granulated or added to the liquid material (B) asadditives may be surface-crosslinked, a water-absorbent resin beforesurface-crosslinking (usually water-absorbent resin obtained only bypolymerization and drying), the water-absorbent resinsurface-crosslinked by the present invention production process, orother surface-crosslinked water-absorbent resin (for example,surface-crosslinking in dispersive systems, such as reversed-phasesuspension). However, particularly, the granulation is carried out orother additive is added preferably by adding the liquid material (B) tothe water-absorbent resin surface-crosslinked by the present inventionproduction process.

Hereinafter, in the production process for a water-absorbent resin,which comprises the step of spray-blending the water-absorbent resin (A)and the liquid material (B) with the blending apparatus equipped withthe spray nozzle (C), with the production process being characterized inthat the liquid material (B) is sprayed from the spray nozzle (C) andits spray pattern is a circular and hollow cone shape (a hollow conespray shape) or a double-convex-lens and elliptic cone shape (a flatspray shape), it is further explained that the modification is carriedout by granulating the water-absorbent resin or blending additives.

When the occasion demands, the water-absorbent resin (A) is granulatedto a granule by use of the liquid material (B) as a binder, and theratio of particles passed through a mesh opening size of 150 μm can bedecreased.

The powder temperature of the above water-absorbent resin (A) ispreferably adjusted to the range of 80 to 35° C., more preferably 70 to35° C., still more preferably 50 to 35° C. Thereafter, the liquidmaterial (B) is blended therewith. In case where the temperature of thewater-absorbent resin (A) before adding the liquid material (B) ishigher, the liquid material (B) is blended ununiformly. In addition,there are disadvantages in adjusting to lower than 35° C., because ittakes much time to forcibly or stationary cool, and besides, theagglomeration of the powder as stationary cooled is observed, and theenergy loss is increased when carrying out reheating.

Water only or an aqueous liquid is preferably used as the binder in viewof efficiency, safety, and cost.

When the aqueous liquid is used as the binder, example thereof includematerials obtained by dissolving the above exemplified hydrophilicorganic solvents and/or water-soluble polymers, such as poly(acrylicacid (salt)), carboxymethyl cellulose, hydroxyethyl cellulose, andpolyethylene glycol.

In addition, for the purpose of modifying by giving various additionalfunctions, additives such as disinfection, deodorization to thewater-absorbing agent, antimicrobial agents, deodorants, perfumes, foodadditives, oxidizing agents, reducing agents, chelating agents,antioxidants, radical inhibitors, and colorants may be added as theliquid material (B) (if necessary, by dissolving or dispersing them insolvents). The above antimicrobial agents, deodorants, perfumes, foodadditives, oxidizing agents, reducing agents, chelating agents,antioxidants, radical inhibitors, and colorants may be added with theaqueous surface-crosslinking agent solution or the binder at the sametime of the surface treatment or the granulation, or may be addedseparately.

The above antimicrobial agents are conventional disinfectant ones, andare not especially limited. Examples thereof include antimicrobialagents shown in JP-A-267500/1999.

In addition, the above deodorants are conventional ones which deodorizeunpleasant components of human urine, such as mercaptan, hydrogensulfide, and ammonia, and are not especially limited. Examples thereofinclude plant extracts from camellias of which deodorizing componentsare, for examples, flavanols or flavonols.

The amount of the binder and/or the additive for the purpose of givingadditional functions to the water-absorbent resin as added can fitly bechanged, depending on the purpose of addition and the kind of theadditive. However, the binder and/or the additive is usually added inthe range of preferably 0.001 to 10 parts by weight, more preferably0.01 to 5 parts by weight, still more preferably 0.05 to 1 part byweight, per 100 parts by weight of the water-absorbent resin (A).

The amount of the solvent (preferably water) of the binder and/or theadditive as used for the purpose of giving additional functions to thewater-absorbent resin is preferably in the range of 1 to 30 parts byweight, more preferably 1 to 10 parts by weight, per 100 parts by weightof the water-absorbent resin (A). In case where the amount as used isless than 1 part by weight, the granulation is insufficient and theadditive is blended ununiformly. In reverse, in case where the amount ismore than 30 parts by weight, a lump having high density is easilycaused and becomes a rigid lump difficult to pulverize. Therefore, thepulverization is necessary in order to adjust to the particle size ofthe produced material (for example, all the particles have a particlesize of less than 1 mm.). However, when the surface-crosslinking iscarried out in the above process, there are disadvantages in that thesurface-crosslinked layer as specially formed might be destroyed bypulverization.

In addition, the liquid temperature of the liquid material (B) ispreferably lower than the powder temperature of the water-absorbentresin (A), more preferably lower than the powder temperature of thewater-absorbent resin (A) by 10° C., still more preferably by 20° C.,most preferably by 30° C. Incidentally, the liquid material (B) issprayed from the spray nozzle (C). Therefore, its liquid temperatureshould be higher than its melting point. In addition, when the liquidtemperature of the liquid material (B) is too high, there aredisadvantages in that: the liquid absorption speed is rapid, and theuniform blending of the liquid material (B) and the water-absorbentresin (A) is inhibited.

The average diameter of the liquid drop of the liquid material (B) asthe binder and/or the additive blended with the water-absorbent resin(A), for the purpose of giving the additional functions to thewater-absorbent resin, is preferably smaller than the average diameterof the water-absorbent resin (A), and is more preferably not larger than300 μm, still more preferably not larger than 250 μm. The averagediameter of the liquid drop is usually in the range of 50 to 200 μm. Incase where the average diameter is larger than 300 μm, there aredisadvantages in that: it is difficult to defuse or disperse the liquidmaterial (B) uniformly; a lump having high density is caused; and theamount of the water-absorbent resin (A) which does not come into contactwith the liquid material (B) is increased in the blending apparatus.

In addition, the spray nozzle (C) is necessary to be fitly selectedaccording to the use condition so that the spray nozzle (C) has apredetermined spray angle. However, the spray angle of the liquidmaterial (B) from the spray nozzle (C) is preferably selected at notless than 50°, more preferably not less than 70°, still more preferablynot less than 90°. In case where the spray angle is less than 50°, theportion where the liquid material (B) is dispersed excessively and theportion where the liquid material (B) is dispersed in low density arecaused in a dispersing state of the liquid material (B) as sprayed inthe blending apparatus, and the partiality is caused in a blending stateof the water-absorbent resin (A) and the liquid material (B). Thewater-absorbent resin (A) as excessively brought into contact with theliquid material (B) produces a lump having high density easily, and itbecomes a rigid lump difficult to pulverize. Therefore, thepulverization is necessary in order to adjust to the particle size ofthe produced material (for example, all the particles have a particlesize of less than 1 mm.). However, when the surface-crosslinking iscarried out in the above process, there are disadvantages in that thesurface-crosslinked layer as specially formed might be destroyed bypulverization. Incidentally, the maximum spray angle is not more than180° because of the structure of the spray.

In addition, there are disadvantages in that the water-absorbent resin(A) as excessively brought into contact with the liquid material (B) iseasily adhered and piled in the blending apparatus in view of the stableoperation of the apparatus. Furthermore, when the portion where theliquid material (B) is excessively dispersed comes into contact with theportion of the blending apparatus, there are disadvantages in that theliquid drop is easily produced and the formation of the piled materialis caused in the apparatus.

On the other hand, there are disadvantages in that the desirablegranulating effect or the additional function is not possibly obtainedin the water-absorbent resin (A) as brought into contact with the liquidmaterial (B) in low density.

Furthermore, when the liquid material (B) is sprayed from the spraynozzle (C) for the purpose of adjusting to the above predetermined sprayangle, the area of the spray-dispersing state of the liquid material (B)projected onto a sectional area which is perpendicular to the axisdirection of the blending apparatus and includes a spraying point of thespray nozzle (C) preferably accounts for 70 to 100% of the sectionalarea perpendicular to the axis direction of the blending apparatus, morepreferably 80 to 100%, still more preferably 90 to 100%. In case wherethe area of the spray-dispersing state of the liquid material (B)projected onto a sectional area which is perpendicular to the axisdirection of the blending apparatus and includes the spraying point ofthe spray nozzle (C) preferably accounts for less than 70% of thesectional area perpendicular to the axis direction of the blendingapparatus, there are disadvantages in that the portion where the liquidmaterial (B) is dispersed excessively and the portion where the liquidmaterial (B) is dispersed in low density are caused in a dispersingstate of the liquid material (B) as sprayed in the blending apparatus,and the partiality is caused in a blending state of the water-absorbentresin (A) and the liquid material (B).

The number of the spray nozzles (C) with which the blending apparatus isequipped may be one or more. The number is preferably two or more sothat the area of the spray-dispersing state projected onto a sectionalarea including the spraying point of the spray nozzle (C) would enlarge.

The blending apparatus as used for blending the water-absorbent resin(A) and the liquid material (B) can be used in the same way as of theabove exemplified blending apparatuses used for blending thewater-absorbent resin (A) with the aqueous surface-crosslinking agentsolution as the aqueous liquid (B).

The resultant mixture in the above process can be dried or heat-treatedif necessary.

The resultant surface-crosslinked or modified water-absorbent resin inthe above way is preferably used because it displays excellentwater-retaining force and higher absorption capacity especially under aload when it is used as a sanitary material.

Step of Heat-Treating

In the present invention, the modification of the water-absorbent resin(A), preferably the crosslinking of its surface neighborhood is carriedout by blending the above-mentioned water-absorbent resin (A) with theliquid material (B) (preferably, and the aqueous surface-crosslinkingagent solution), and thereafter heat-treating the resultant mixture.Incidentally, the modification of the water-absorbent resin means thegranulation of water-absorbent resins or the addition of additives, andfurther, examples of its modes include the surface-crosslinking by theaddition of the surface-crosslinking agent. The above mentionedheat-treatment depends upon the surface-crosslinking agent as used, butis preferably carried out at a water-absorbent resin temperature(material temperature) or heat medium temperature of 60 to 250° C., morepreferably 80 to 250° C., still more preferably 100 to 230° C.,particularly preferably 150 to 200° C. In case where the treatingtemperature is lower than 60° C., the uniform crosslinked structure isnot formed. Accordingly, there are disadvantages in that the crosslinkedwater-absorbent resin having high absorption capacity under a loadcannot be obtained. In addition, the productivity is caused to lowerbecause it takes much time to carry out the heat treatment. In casewhere the treating temperature is higher than 250° C., thewater-absorbent resin (A) is caused to deteriorate. Accordingly, thereare disadvantages in that the propertied of the surface-crosslinkedwater-absorbent resin are lowered. Incidentally, the above-mentionedtreating temperature is preferably the water-absorbent resin temperature(material temperature) in order to control the surface-crosslinkingreaction exactly.

In addition, even if the surface-crosslinking agent is not used, theheat treatment is preferably carried out at the above-mentionedtemperature in order to uniformly diffuse the liquid material (B) and toimprove the granulation strength of the water-absorbent resin. Inaddition, the heat treatment may be carried out by spraying with ablender having a function of heating, or the heating and spraying may becarried out at the same time.

In the present invention, the liquid material (B) is sprayed and theabove-mentioned heat treatment is carried out. Furthermore, theatmosphere of the upper space inside of the heat-treating apparatus isalso preferably adjusted to a specific range when the heat treatment iscarried out.

In a heat-treating method which involves adding a liquid material to awater-absorbent resin, the reaction or modification was controlled bythe water-absorbent resin temperature (material temperature) or heatmedium temperature in the past. However, as long as the water-absorbentresin temperature (material temperature) or heat medium temperature wasmerely determined, the improvement of the properties might beinsufficient, and the properties might not be stabilized in a continuousproduction process. The present inventors diligently studied to solvethese problems. As a result, in order to improve or stabilize theproperties caused by heat treatment, they solved these problems bycontrolling the upper space condition inside of the heat-treatingapparatus while heat-treating to a specific atmosphere, wherein theupper space did not draw any attention in the past.

The present invention is accomplished by heat-treating thewater-absorbent resin powder as obtained in the above way under anatmosphere inside of the upper space of the heat-treating apparatushaving a dew point of not higher than 60° C. and a temperature of notlower than 90° C., preferably crosslinking the water-absorbent resinpowder surface in the presence of a hydrophilic solution including theaforementioned crosslinking agent or its aqueous solution to preferablycarry out a crosslinking reaction. Incidentally, the atmosphere in thepresent invention means the temperature and dew point of the upper spaceinside of the heat-treating apparatus, wherein the upper space insideincludes the water-absorbent resin powder, and the temperature of theheat-treating apparatus may be equal to or different from theatmosphere.

In case where the water-absorbent resin powder having a water content ofnot less than 10 weight % is used, there are disadvantages in that: notonly the aimed properties are not obtained but also much energy isrequired to obtain the atmosphere having a dew point of not higher than60° C. and a temperature of not lower than 90° C.

In case where the dew point is not higher than 60° C. and thetemperature is not higher than 90° C., even if the water-absorbent resintemperature (material temperature) or heat medium temperature issufficient, the crosslinking reaction between the carboxyl group on thesurface of the water-absorbent resin powder and the crosslinking agentis not carried out sufficiently, and the amount of the unreactedcrosslinking agent might be increased. In addition, in case where thedew point is not lower than 60° C. even if the temperature is not lowerthan 90° C., even if the water-absorbent resin temperature (materialtemperature) or heat medium temperature is sufficient, the crosslinkingagent permeates into the internal portion of the water-absorbent resinpowder particles, and it might be difficult to carry out thecrosslinking reaction between the carboxyl group on the surface of thewater-absorbent resin powder and the crosslinking agent, because thewater content is slowly evaporated from the water-absorbent resinpowder. Therefore, as is shown in FIG. 4, the dew point and thetemperature are adjusted to not higher than 60° C. and not lower than90° C. respectively in order to maintain the permeation of thecrosslinking agent into the surface neighborhood of the water-absorbentresin powder particles in an optimum state, and to make the surface ofthe water-absorbent resin powder a necessary and adequate crosslinkingstate. It is found that the influence of the temperature and the dewpoint in the atmosphere especially has a great influence on the heattreatment in the surface-crosslinking, especially the heat treatmentwith the above-mentioned specific crosslinking agent, and further theheat treatment with the polyhydric alcohol.

As to the heat-treating apparatus to treat the water-absorbent resinpowder in the above condition, conventional dryers or furnaces equippedwith a gas supplying or exhausting apparatus to make the predeterminedatmosphere can be used. The usable gas is vapor, air, and nitrogen, andis preferably air. The amount as supplied is fitly determined. The gasfor adjusting the temperature or dew point may fitly be under reduced orcompressed pressure, or may fitly be heated or cooled. Air having anearly room temperature (for example, 0 to 50° C.) may be supplied at asubstantially ordinary pressure (1.013×10⁵ P a (1 atm)±10%, preferably±5%, more preferably ±1%). For example, conductive-heat-transfer-type,radiative-heat-transfer-type, hot-wind-heat-transfer-type, ordielectric-heating-type dryers or furnaces equipped with a gas supplyingor exhausting apparatus are favorable. Examples thereof includebelt-type, channel-blending-type, rotary-type, disk-type, kneading-type,fluidized-bed-type, air-blow-type, infrared-type, or electron-beam-typedryers or furnaces equipped with an apparatus for supplying a mixed gasof air and/or an inert gas. The temperature of these heat-treatingapparatuses may be equal to or different from that of the atmosphere ofthe upper space inside of the heat-treating apparatus, but is usuallyadjusted to the range of 110 to 250° C., preferably 150 to 210° C. Inaddition, the heat-treating apparatus is heated and its temperature isadjusted higher than the temperature of the atmosphere of the upperspace inside of the heat-treating apparatus by 0 to 120° C., preferably30 to 100° C.

Among these heat treatments, preferably by the conductive-heat-transferor hot-wind-heat-transfer, more preferably by theconductive-heat-transfer, the water-absorbent resin is heated whilebeing stirred or fluidized, and the atmosphere of the upper space of thetreating apparatus may also be controlled, wherein the atmosphere didnot draw any attention in the past. When the water-absorbent resin isheated by the conductive-heat-transfer, the water-absorbent resin isheated through the heat-transfer-surface (for example, sidewall orstirring blade of paddle-type dryer) heated by heat medium, and theupper space of the water-absorbent resin not in contact with theheat-transfer-surface may be controlled to a specific temperature and aspecific dew point. Incidentally, in the present invention whichinvolves heat-treating at the above specific dew point, the liquidmaterial is preferably added by spray, more preferably added with theabove spray pattern, and further, the resultant mixture is heat-treated.The amount as treated is not influenced by the scale of the apparatus(scale factor) even if continuously carrying out the heat treatment of10 kg/hr, further 100 kg/hr, further more 1,000 kg/hr, still furthermore 2,000 kg/hr, particularly further more 3,000 kg/hr, and the liquidmaterial is fitly used.

Incidentally, the present invention may have a mode including either orboth of the step of spray-blending the liquid material and the step ofheat-treating. The effect of the present invention can be displayed ifthe mode includes at least one selected from the group consisting of thestep of spray-blending the liquid material and the step ofheat-treating.

(Water-absorbent Resin According to the Present Invention):

The water-absorbent resin, according to the present invention, ispreferably obtained by the production process according to the presentinvention, but is not limited thereto.

The water-absorbent resin, according to the present invention, issurface-crosslinked with a surface-crosslinking agent including at leasta polyhydric alcohol, has a particle size distribution such that theratio of particles having particle diameters of smaller than 150 μm isnot more than 5 weight %, and exhibits an absorption capacity without aload of not less than 30 g/g, with the water-absorbent resin beingcharacterized in that: the single-layer absorption capacity (10 min.) ofparticles having particle diameters of 600 to 300 μm is not less than 30g/g under a load; the single-layer absorption capacity (60 min.) ofparticles having particle diameters of 600 to 300 μm is not less than 30g/g under a load; the single-layer absorption capacity (10 min.) ofparticles having particle diameters of 300 to 150 μm is not less than 30g/g under a load; and the single-layer absorption capacity (60 min.) ofparticles having particle diameters of 300 to 150 μm is not less than 30g/g under a load.

It is necessary that the water-absorbent resin according to the presentinvention is surface-crosslinked with the surface-crosslinking agentincluding at least a polyhydric alcohol. If the water-absorbent resin isnot surface-crosslinked in the above way, the blendability with fibermaterials or shape-preserving ability is deteriorated when thewater-absorbent resin is used for sanitary materials, there is apossibility that the water-absorbent resin displaying the followingindex of uniform surface-treatment cannot be obtained, and theabsorption capacity is caused to lower when the water-absorbent resin isused as sanitary materials such as diapers.

It is necessary that the water-absorbent resin according to the presentinvention has a particle size distribution such that the ratio ofparticles having particle diameters of smaller than 150 μm is not morethan 5 weight %. In case where the water-absorbent resin has a particlesize distribution such that the ratio of particles having particlediameters of smaller than 150 μm is more than 5 weight %, an opening inabsorbing materials is clogged with the particles having particlediameters of smaller than 150 μm when the water-absorbent resin is usedas sanitary materials such as diapers. Therefore, liquids are inhibitedfrom diffusing, and the properties of the product are caused to lower.

It is necessary that the water-absorbent resin according to the presentinvention exhibits an absorption capacity without a load of not lessthan 30 g/g. In case where the water-absorbent resin exhibits anabsorption capacity without a load of less than 30 g/g, it isuneconomical because a large amount of the water-absorbent resin isnecessary to obtain desirable absorption capacity when thewater-absorbent resin is used as sanitary materials such as diapers.

When a water-absorbent resin is practically used for diapers, it isnecessary that its particle each displays excellent capacities in orderthat the water-absorbent resin may realize excellent capacities.However, conventional measurement methods lack methods for estimatingcapacity of one of its particles.

For example, among the conventional measurement methods, the absorptioncapacity under a load described in the present specification was anestimate for the entirety of water-absorbent resin particles having aparticle size distribution. Therefore, it is difficult to estimatecapacities of each particle. In addition, even if the absorptioncapacity under a load was measured after adjustment of particlediameters (for example, in the range of 600 to 300 μm), the estimate wasfor a single particle diameter range (U.S. Pat. No. 5,147,343B1, EP5,320,02B1, and U.S. Pat. No. 5,601,542B1). Therefore, thesurface-crosslinking state of the single particle diameter range couldnot be compared to that of other particle diameter ranges.

Furthermore, when the measurement of the absorption capacity under aload was carried out before, the amount of the water-absorbent resin asspread was much. Therefore, gels are in a tiering state after swelling,and a factor such as rearrangement of gels while swelling is included inaddition to swellability of the water-absorbent resin under a load. Inaddition, vacancy liquid existing between the swollen gel particles,inhibits the estimate of the properties of the water-absorbent resinitself. In order to exclude this factor, the estimate, such that theamount as spread is adjusted in order that a gel layer can be singleeven after swelling, and then the vacancy liquid is removed, is asingle-layer absorption capacity under a load. Its concrete measurementmethod is explained in the following examples.

The water-absorbent resin according to the present invention ispreferably obtained by the production process for a water-absorbentresin according to the present invention. The production process ischaracterized in that: the treatment, preferably surface-crosslinkingtreatment of each water-absorbent resin particle is carried out highlyuniformly. Therefore, the estimate represented by the single-layerabsorption capacity under a load can exactly reflect capacities of thewater-absorbent resin according to the present invention.

The water-absorbent resin, according to the present invention, ischaracterized in that: the single-layer absorption capacity (10 min.) ofparticles having particle diameters of 600 to 300 μm is not less than 30g/g under a load; the single-layer absorption capacity (60 min.) ofparticles having particle diameters of 600 to 300 μm is not less than 30g/g under a load; the single-layer absorption capacity (10 min.) ofparticles having particle diameters of 300 to 150 μm is not less than 30g/g under a load; and the single-layer absorption capacity (60 min.) ofparticles having particle diameters of 300 to 150 μm is not less than 30g/g under a load. The above-mentioned respective single-layer absorptioncapacities are preferably not less than 31 g/g under a load, morepreferably not less than 32 g/g. In case where the above-mentionedrespective single-layer absorption capacities are less than 30 g/g undera load, there are disadvantages in that the uniform treatment might notbe carried out sufficiently.

The water-absorbent resin, according to the present invention, has aparticle size distribution such that: the ratio of particles havingparticle diameters of 600 to 300 μm is preferably in the range of 65 to85 weight %, more preferably 70 to 80 weight %; and the ratio ofparticles having particle diameters of 300 to 150 μm is preferably inthe range of 10 to 30 weight %, more preferably 15 to 25 weight %.

In the water-absorbent resin according to the present invention, thetime variation of the single-layer absorption capacity of particleshaving particle diameters of 600 to 300 μm under a load is preferablynot less than 0.80.

Then, the time variation of the single-layer absorption capacity ofparticles having particle diameters of 600 to 300 μm under a load iscalculated according to the following equation, and a value representingswellability under a load. This time variation is more preferably notless than 0.85, still more preferably not less than 0.90. That is tosay, if the time variation is close to 1, there are advantages inreaching saturated swell in a short time.

Time variation of single-layer absorption capacity of particles havingparticle diameters of 600 to 300 μm under a load=(single-layerabsorption capacity (10 min.) of particles having particle diameters of600 to 300 μm under a load)/(single-layer absorption capacity (60 min.)of particles having particle diameters of 600 to 300 μm under a load).

In the water-absorbent resin according to the present invention, thetime variation of the single-layer absorption capacity of particleshaving particle diameters of 300 to 150 μm under a load is preferablynot less than 0.90.

Then, the time variation of the single-layer absorption capacity ofparticles having particle diameters of 300 to 150 μm under a load iscalculated according to the following equation, and a value representingswellability under a load. This time variation is more preferably notless than 0.92, still more preferably not less than 0.95. That is tosay, if the time variation is close to 1, there are advantages inreaching saturated swell in a short time.

Time variation of single-layer absorption capacity of particles havingparticle diameters of 300 to 150 μm under a load=(single-layerabsorption capacity (10 min.) of particles having particle diameters of300 to 150 μm under a load)/(single-layer absorption capacity (60 min.)of particles having particle diameters of 300 to 150 μm under a load).

In the water-absorbent resin according to the present invention, thevariation between particles of the single-layer absorption capacity (10min.) under a load is preferably in the range of 0.90 to 1.10.

Then, the variation between particles of the single-layer absorptioncapacity (10 min.) under a load is calculated according to the followingequation, and a value representing uniformity of blending state. Thisvariation between particles is more preferably in the range of 0.95 to1.05, still more preferably 0.97 to 1.03.

Variation between particles of the single-layer absorption capacity (10min.) under a load=(single-layer absorption capacity (10 min.) ofparticles having particle diameters of 300 to 150 μm under aload)/(single-layer absorption capacity (10 min.) of particles havingparticle diameters of 600 to 300 μm under a load).

In the water-absorbent resin according to the present invention, thevariation between particles of the single-layer absorption capacity (60min.) under a load is preferably in the range of not less than 0.90.

Then, the variation between particles of the single-layer absorptioncapacity (60 min.) under a load is calculated according to the followingequation, and a value representing uniformity of blending state. Thisvariation between particles is more preferably not less than 0.92, stillmore preferably not less than 0.95.

Variation between particles of the single-layer absorption capacity (60min.) under a load=(single-layer absorption capacity (60 min.) ofparticles having particle diameters of 300 to 150 μm under aload)/(single-layer absorption capacity (60 min.) of particles havingparticle diameters of 600 to 300 μm under a load).

The water-absorbent resin, according to the present invention, issurface-crosslinked with a surface-crosslinking agent including at leasta polyhydric alcohol, has a particle size distribution such that theratio of particles having particle diameters of smaller than 150 μm isnot more than 5 weight %, and exhibits an absorption capacity without aload of not less than 30 g/g, with the water-absorbent resin beingcharacterized in that the index of uniform surface-treatment is not lessthan 0.70.

Then, the index of uniform surface-treatment is calculated according tothe following equation, and a value enabling to exactly representuniform surface-treatment. The index of uniform surface-treatment ispreferably not less than 0.72, more preferably not less than 0.75, stillmore preferably not less than 0.80. If the index is close to 1, thereare advantages in that the uniformity is enhanced.

Index of uniform surface-treatment=(time variation of single-layerabsorption capacity of particles having particle diameters of 600 to 300μm under a load)×(time variation of single-layer absorption capacity ofparticles having particle diameters of 300 to 150 μm under aload)×(variation between particles of the single-layer absorptioncapacity (10 min.) under a load)=(variation between particles of thesingle-layer absorption capacity (60 min.) under a load).

In the water-absorbent resin according to the present invention, the Lvalue of light index measured with such as a spectrophotometer ispreferably not less than 85, and the a value and b value representingchromaticness index are preferably in the range of −2 to 2, and 0 to 9respectively. In case where the L, a, and b values is beyond theseranges, there are disadvantages in that the uniform treatment which is acharacteristic of the present invention might not be carried out.

The water-absorbent resin, according to the present invention, canpreferably be used as sanitary materials, such as disposable diapers,sanitary napkins, and incontinent pads due to its excellent properties,and provides the sanitary material according to the present invention.

Water-absorbent resins are generally produced or used as powders.Therefore, there was a problem such that the properties of the resultantsanitary material varied due to bias (segregation) of particle diametersof the powders, and the properties values were changed depending uponthe absorption time. However, the present invention water-absorbentresin includes the polyhydric alcohol, and have higher properties(higher absorption capacity), and further, there is no difference of theproperties between particle diameters or absorption times. Therefore, itis favorable when the water-absorbent resin is use as a sanitarymaterial. When using the water-absorbent resin as a sanitary material,it was found that the properties values (single-layer absorptioncapacity under a load for a specific particle diameter or specificabsorption time) of the present invention are critically importantvalues. The present invention water-absorbent resin has higherproperties, and can be used in higher resin concentration such that thecore concentration defined by fiber material/water-absorbent resin is inthe range of 30 to 100%, preferably 40 to 100%, more preferably 50 to100%.

Effects and Advantages of the Invention

According to the present invention, the uniform blending of awater-absorbent resin with a liquid material, which is thoughtconventionally difficult because of the character such that thewater-absorbent resin rapidly absorbs the liquid material when thewater-absorbent resin comes into contact with the liquid material, canbe carried out extremely easily and stably for a long time.

According to the present invention, the water-absorbent resin powder isefficiently and effectively allowed to react with the crosslinkingagent. Therefore, there are advantages in view of industry and economy.The water-absorbent resin resultant from the surface-treatment of thewater-absorbent resin powder in the above way has excellent absorptioncapacity and absorption capacity under a load.

Therefore, the water-absorbent resin resultant from thesurface-treatment according to the present invention can fitly be usedas water-absorbent resins for sanitary materials, such as disposablediapers and sanitary cotton, and besides, dew condensation inhibitorsfor building materials, water preserving agents for agriculture andgardening, or drying agent.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention is more specifically illustrated bythe following examples of some preferred embodiments in comparison withcomparative examples not according to the invention. However, thepresent invention is not limited to the below-mentioned examples.Incidentally, absorption capacity without load and absorption capacityunder a load are measured in the following way.

(a) Absorption Capacity Without Load (Merely Referred as AbsorptionCapacity):

A nonwoven fabric bag (60 mm×60 mm), in which about 0.20 g of awater-absorbent resin was put uniformly, and was immersed into anaqueous sodium chloride solution of 0.9 wt % (physiological salinesolution) at room temperature (25±2° C.). After 30 minutes, the bag wasdrawn up and then drained at 250×9.81 m/sec² (250 G) with a centrifugefor 3 minutes. Then, weight W₁ (g) of the bag was measured. In addition,the same procedure as the above was carried out using no water-absorbentresin, and weight W₂ (g) of the resultant bag was measured.

Thus, the absorption capacity (g/g) without load was calculated fromthese weights W₁ and W₂ in accordance with the following equation:

Absorption capacity (g/g) without load=(weight W ₁ (g)−weight W ₂(g))/weight of water-absorbent resin (g).

(b) Absorption Capacity Under Load (Swollen Liquid: Physiological SalineSolution):

Hereinafter, when a physiological saline solution is used as a swollenliquid, a measurement apparatus as used for measuring the absorptioncapacity under a load is briefly explained while referring to FIG. 3.

As is shown in FIG. 3, the measurement apparatus comprises: a balance 1;a vessel 2 of a predetermined capacity as mounted on the balance 1; anair-inhaling pipe 3; an introducing tube 4; a glass filter 6; and ameasurement part 5 as mounted on this glass filter 6.

The vessel 2 has an opening part 2 a on the top and an opening part 2 bon the side, respectively. The air-inhaling pipe 3 is inserted in theopening part, and the introducing tube 4 is fitted to the opening part 2b. Incidentally, the vessel 2 contains a predetermined amount of anaqueous sodium chloride solution of 0.9 wt % (physiological salinesolution, liquid temperature: 25±2° C.) 11.

In addition, the lower part of the air-inhaling pipe 3 is submerged inthe physiological saline solution 11. The air-inhaling pipe 3 is fittedto keep internal pressure of the vessel 2 almost atmospheric.

The glass filter 6 is formed in a diameter of 70 mm. The vessel 2 andthe glass filter 6 are connected to each other through the introducingtube 4 made of a silicone resin. In addition, the position and the levelof the glass filter 6 are fixed relative to the vessel 2. Furthermore,the glass filter 6 is fixed so that the position of the upper surface ofthe glass filter 6 would be slightly higher than the lower end of theair-inhaling pipe 3.

The measurement part 5 comprises: a filter paper 7; a supportingcylinder 8; a wire net 9 as attached to the bottom of the supportingcylinder 8; and a weight 10. The measurement part 5 is formed bymounting the filter paper 7 and the supporting cylinder 8, namely a wirenet 9, in this order on the glass filter 6 and further mounting theweight 10 inside the supporting cylinder 8, namely on the wire net 9.The supporting cylinder 8 is formed in an internal diameter of 60 mm.The wire net 9 is made of stainless steel and formed in a mesh size of38 μm (400 mesh). An arrangement is made such that a predeterminedamount of water-absorbent resin can uniformly be spread on the wire net9. In addition, the weight 10 is adjusted in weight such that a load of20 g/cm² (about 1.96 kPa) can uniformly be applied to thewater-absorbent resin.

The absorption capacity under a load was measured with the measurementapparatus having the above-mentioned constitution. The measurementmethod is hereinafter explained.

First, predetermined preparatory operations were made, in which, forexample, a predetermined amount of the physiological saline solution 11was placed into the vessel 2, and the air-inhaling pipe 3 was insertedinto the vessel 2. Next, the filter paper 7 was mounted on the glassfilter 6, and further. In parallel with this mounting operation, 0.900 gof water-absorbent resin was uniformly spread inside the supportingcylinder 8, namely, on the wire net 9, and the weight 10 was then put onthis water-absorbent resin.

Then, the wire net 9 of the supporting cylinder 8, on which thewater-absorbent resin and the weight 10 were put, was mounted on thefilter paper 7 concentrically with the glass filter 6.

Then, the weight W₃ (g) of the physiological saline solution 11, asabsorbed by the water-absorbent resin over a period of 60 minutes sincethe supporting cylinder 8 was mounted on the filter paper 7, wasdetermined from a measured value with the balance 1.

Then, the absorption capacity (g/g) under the load in 60 minutes fromthe absorption initiation was calculated from the above-mentioned W₃ (g)and the weight of the water-absorbent resin (0.900 g) in accordance withthe following equation.

Absorption capacity (g/g) under load=weight W ₃(g)/weight (g) ofwater-absorbent resin

(c) Absorption Capacity Under Load (Swollen Liquid: Synthetic Urine):

Hereinafter, when synthetic urine is used as a swollen liquid, ameasurement apparatus as used for measuring the absorption capacityunder a load is briefly explained while referring to FIG. 3.

As is shown in FIG. 3, the measurement apparatus comprises: a balance 1;a vessel 2 of a predetermined capacity as mounted on the balance 1; anair-inhaling pipe 3; an introducing tube 4; a glass filter 6; and ameasurement part 5 as mounted on this glass filter 6. The vessel 2 hasan opening part 2 a on the top and an opening part 2 b on the side,respectively. The air-inhaling pipe 3 is inserted in the opening part,and the introducing tube 4 is fitted to the opening part 2 b. Inaddition, the vessel 2 contains a predetermined amount of syntheticurine 11. The lower part of the air-inhaling pipe 3 is submerged in thesynthetic urine 11. The glass filter 6 is formed in a diameter of 70 mm.The vessel 2 and the glass filter 6 are connected to each other throughthe introducing tube. In addition, the upper portion of the glass filter6 is fixed so that the position of the upper surface of the glass filter6 would be slightly higher than the lower end of the air-inhaling pipe3. The measurement part 5 comprises: a filter paper 7; a supportingcylinder 8; a wire net 9 as attached to the bottom of the supportingcylinder 8; and a weight 10. Then the measurement part 5 is formed bymounting the filter paper 7 and the supporting cylinder 8, namely a wirenet 9, in this order on the glass filter 6 and further mounting theweight 10 inside the supporting cylinder 8, namely on the wire net 9.The supporting cylinder 8 is formed in an internal diameter of 60 mm.The wire net 9 is made of stainless steel and formed in a mesh size of400 mesh (38 μm). An arrangement is made such that a predeterminedamount of water-absorbent resin can uniformly be spread on the wire net9. In addition, the weight 10 is adjusted in weight such that a load of50 g/cm² (about 4.83 kPa) can uniformly be applied to thewater-absorbent resin.

The absorption capacity under a load was measured with the measurementapparatus having the above-mentioned constitution. The measurementmethod is hereinafter explained.

First, predetermined preparatory operations were made, in which, forexample, a predetermined amount of the synthetic urine 11 was placedinto the vessel 2, and the air-inhaling pipe 3 was inserted into thevessel 2. Next, the filter paper 7 was mounted on the glass filter 6,and further. In parallel with this mounting operation, 0.900 g ofwater-absorbent resin was uniformly spread inside the supportingcylinder, namely, on the wire net 9, and the weight 10 was then put onthis water-absorbent resin. Then, the wire net 9 of the supportingcylinder 8, on which the water-absorbent resin and the weight 10 wereput was mounted.

Then, the weight W₄ (g) of the synthetic urine 11, as absorbed by thewater-absorbent resin over a period of 60 minutes since the supportingcylinder 8 was mounted on the filter paper 7, was determined from ameasured value with the balance 1.

Then, the absorption capacity (g/g) under the load in 60 minutes fromthe absorption initiation was calculated from the above-mentioned W₄ (g)in accordance with the following equation:

Absorption capacity (g/g) under load=weight W ₄ (g)/weight (g) ofwater-absorbent resin

The composition of the synthetic urine is shown in the following.

(sodium sulfate: 0.2 weight %, potassium chloride: 0.2 weight %,magnesium chloride 6 hydrate: 0.05 weight %, calcium chloride dihydrate:0.025 weight %, ammonium dihydrogen phosphate: 0.085 weight %,diammonium hydrogen phosphate: 0.015 weight %, and deionized water:99.425 weight %)

(d) Estimate of Uniform Surface-Treatment:

A measurement apparatus as used for measuring the estimate of uniformsurface-treatment is equal to that as used for measuring the absorptioncapacity under a load shown in FIG. 3

The estimate of uniform surface-treatment is carried out with thisapparatus. Its measurement method is explained in the following.

First, predetermined preparatory operations were made, in which, forexample, a predetermined amount of synthetic urine 11 (comprising: 0.2weight % of sodium sulfate, 0.2 weight % of potassium chloride, 0.05weight % of magnesium chloride 6 hydrate, 0.025 weight % of calciumchloride dihydrate, 0.085 weight % of ammonium dihydrogen phosphate,0.015 weight % of diammonium hydrogen phosphate, and 99.425 weight % ofdeionized water, and liquid temperature: 25±2° C.) was placed into avessel 2, and an air-inhaling pipe 3 was inserted into the vessel 2.Next, a filter paper 7 was mounted on a glass filter 6. In parallel withthis mounting operation, 0.055±0.005 g of water-absorbent resin wasuniformly spread inside a supporting cylinder 8, namely, on a wire net9, and a weight 10 is then put on this water-absorbent resin.Thereafter, the total weight before measurement W₅ (g) of the supportingcylinder 8 fixed by the wire net 9, the water-absorbent resin, and theweight 10 was measured. Incidentally, as to the water-absorbent resinfor measuring the estimate of uniform surface-treatment, water-absorbentresins having particle diameters of 600 to 300 μm and 300 to 150 μmrespectively obtained by beforehand classification were used asmeasurement samples.

Then, the wire net 9 of the supporting cylinder 8, on which thewater-absorbent resin and the weight 10 were put, was mounted on thefilter paper 7 concentrically with the glass filter 6.

Then, the synthetic urine 11 was absorbed by the water-absorbent resinover a period of 10 or 60 minutes since the supporting cylinder 8 wasmounted on the filter paper 7.

After a predetermined minute passed, the supporting cylinder 8 wassoftly transferred on five pieces of filter papers (made by AdvantechToyo, No. 2, diameter: 90 mm) as prepared beforehand while the load wasapplied to the water-absorbent resin without removing the weight 10, andvacancy liquid between the gelled water-absorbent resin particles asabsorbed was drawn out for 2 minutes. The reason while the load wasapplied to the water-absorbent resin without removing the weight 10 inthe above way and the vacancy liquid was drawn out is because thewater-absorbent is inhibited from absorbing the vacancy liquid betweenthe particles by lightening the weight.

Then, the total weight after measurement W₆ (g) of the supportingcylinder 8 fixed by the wire net 9, the water-absorbent resin, and theweight 10 was measured.

Then, the single-layer absorption capacity (g/g) under the load in 10 or60 minutes from the absorption initiation was calculated from theabove-mentioned W₅ (g) and W₆ (g) in accordance with the followingequation.

Single-layer absorption capacity (g/g) under load=(total weight aftermeasurement W ₆(g)−total weight before measurement W ₅(g))/weight (g) ofwater-absorbent resin

Accordingly, the following four values of single-layer absorptioncapacities (g/g) under a load were calculated: the single-layerabsorption capacity (10 min.) of particles having particle diameters of600 to 300 μm under a load; the single-layer absorption capacity (60min.) of particles having particle diameters of 600 to 300 μm under aload; the single-layer absorption capacity (10 min.) of particles havingparticle diameters of 300 to 150 μm under a load; and the single-layerabsorption capacity (60 min.) of particles having particle diameters of300 to 150μ under a load.

When the estimate of uniform surface-treatment was measured, the timevariation of the single-layer absorption capacity under a load and thevariation between particles of the single-layer absorption capacityunder a load were further calculated.

The time variation of single-layer absorption capacity of particleshaving particle diameters of 600 to 300 μm under a load was calculatedin accordance with the following equation.

Time variation of single-layer absorption capacity of particles havingparticle diameters of 600 to 300 μm under a load=(single-layerabsorption capacity (10 min.) of particles having particle diameters of600 to 300 μm under a load)/(single-layer absorption capacity (60 min.)of particles having particle diameters of 600 to 300 μm under a load).

The time variation of single-layer absorption capacity of particleshaving particle diameters of 300 to 150 μm under a load was calculatedin accordance with the following equation.

Time variation of single-layer absorption capacity of particles havingparticle diameters of 300 to 150 μm under a load=(single-layerabsorption capacity (10 min.) of particles having particle diameters of300 to 150 μm under a load)/(single-layer absorption capacity (60 min.)of particles having particle diameters of 300 to 150 μm under a load).

The variation between particles of the single-layer absorption capacity(10 min.) under a load was calculated in accordance with the followingequation.

Variation between particles of the single-layer absorption capacity (10min.) under a load=(single-layer absorption capacity (10 min.) ofparticles having particle diameters of 300 to 150 μm under aload)/(single-layer absorption capacity (10 min.) of particles havingparticle diameters of 600 to 300 μm under a load).

The variation between particles of the single-layer absorption capacity(60 min.) under a load was calculated in accordance with the followingequation.

Variation between particles of the single-layer absorption capacity (60min.) under a load=(single-layer absorption capacity (60 min.) ofparticles having particle diameters of 300 to 150 μunder aload)/(single-layer absorption capacity (60 min.) of particles havingparticle diameters of 600 to 300 μm under a load).

Furthermore, the index of uniform surface-treatment was calculated fromthe four values of the variations as calculated in the above way inaccordance with the following equation.

Index of uniform surface-treatment=(time variation of single-layerabsorption capacity of particles having particle diameters of 600 to 300μm under a load)×(time variation of single-layer absorption capacity ofparticles having particle diameters of 300 to 150 μm under aload)×(variation between particles of the single-layer absorptioncapacity (10 min.) under a load)×(variation between particles of thesingle-layer absorption capacity (60 min.) under a load).

EXAMPLE 1

In a kneader equipped with two sigma type blades, an aqueous acrylicacid salt monomer solution having a monomer concentration of 38 weight %and a neutralization ratio of 75 mol % was prepared, wherein the aqueousmonomer solution comprised an aqueous sodium acrylate solution, acrylicacid and water. Trimethylolpropane triacrylate as aninternal-crosslinking agent was dissolved therein so that itsconcentration would be adjusted to 0.02 mol % of the monomer. Next, theamount of dissolved oxygen of the aqueous monomer solution was decreasedand the entirety of the reaction apparatus was replaced with nitrogengas by introducing the nitrogen gas into the aqueous solution. Next,while the two sigma blades were rotated, 0.05 mol % of sodium persulfateand 0.0003 mol % of L-ascorbic acid were added as a polymerizationinitiator to carry out a stirring polymerization in the kneader, thusobtaining a hydrogel polymer having an average particle diameter ofabout 2 mm after 40 minutes.

The resultant hydrogel polymer was dried in a hot air dryer adjusted ata temperature of 170° C. for 60 minutes. The resultant dried product waspulverized with a roller mill, and then classified with a mesh of 850 μmto remove particles larger than 850 μm, thus obtaining a water-absorbentresin (A1).

The above water-absorbent resin (A1) was kept at about 60° C. andsupplied into a continuous high-speed-stirring blender (a turbulizermade by Hosokawa Micron Co., Ltd.) equipped with two hydraulic hollowcone spray nozzles (C1, 1/4M-K-040, made by H. Ikeuchi & Co., Ltd.;their spray patterns were circular and hollow cone shapes) with afeeding speed of 100 kg/hr, and an aqueous surface-crosslinking agentsolution prepared with a blending ratio of glycerin:water:isopropylalcohol=1:4:1 as a liquid material (B1) was blended therewith byspraying so that the amount of the aqueous solution as added would beadjusted to 3 weight % relative to the weight of the water-absorbentresin (A1). After the resultant mixture was heat-treated at awater-absorbent resin temperature (material temperature) of 190° C. forone hour, the entirety was passed through a sieve having a mesh openingof 850 μm, thus obtaining a surface-treated water-absorbent resin (1).

The hydraulic hollow cone spray nozzles (C1, 1/4M-K-040) were used.Therefore, the spray angle of the aqueous surface-treating agentsolution from the hydraulic hollow cone spray nozzles was 70°, and thedispersing area of a spray-dispersing state projected onto a sectionalarea perpendicular to the stirring shaft direction of the blendingapparatus accounted for about 89% by the nozzles.

After finishing the above procedure, the internal portion of the blenderwas observed. Then, large piled materials were observed little.

The properties of the resultant surface-treated water-absorbent resin(1) were listed in Table 1.

EXAMPLE 2

A surface-treated water-absorbent resin (2) was obtained in the same wayas of Example 1 except that the blender was changed into a continuoushigh-speed-stirring blender equipped with two hydraulic flat spraynozzles (C2, 1/4M-V-115-05, made by H. Ikeuchi & Co., Ltd.; their spraypatterns were double-convex-lens and elliptic cone shapes).Incidentally, the hydraulic flat spray nozzles (C2, 1/4M-V-115-05) wereattached to the blender very carefully so that the spray angle would bethe largest when the dispersing area of a spray-dispersing state wasprojected onto a sectional area perpendicular to the stirring shaftdirection of the continuous high-speed-stirring blending apparatus.

The spray angle was 110° by use of the hydraulic flat spray nozzles (C2,1/4M-V-115-05) when the dispersing area of a spray-dispersing state wasprojected onto a sectional area perpendicular to the stirring shaftdirection of the blending apparatus. The dispersing area of aspray-dispersing state projected onto a sectional area perpendicular tothe shaft direction of the blending apparatus accounted for about 97% bythe nozzles.

After finishing the above procedure, the internal portion of the blenderwas observed. Then, large piled materials were observed little.

The properties of the resultant surface-treated water-absorbent resin(2) were listed in Table 1.

EXAMPLE 3

A surface-treated water-absorbent resin (3) was obtained in the same wayas of Example 1 except that the blender was changed into a continuoushigh-speed-stirring blender equipped with a hydraulic hollow cone spraynozzle (C3, 1/4M-K-100, made by H. Ikeuchi & Co., Ltd.; its spraypattern was a circular and hollow cone shape (a hollow cone sprayshape)).

The hydraulic hollow cone spray nozzle (C3, 1/4M-K-100) was used.Therefore, the spray angle of the aqueous surface-treating agentsolution from the hydraulic hollow cone spray nozzle was 70°, and thedispersing area of a spray-dispersing state projected onto a sectionalarea perpendicular to the shaft direction of the blending apparatusaccounted for about 77%.

After finishing the above procedure, the internal portion of the blenderwas observed. Then, large piled materials were observed little.

The properties of the resultant surface-treated water-absorbent resin(3) were listed in Table 1.

COMPARATIVE EXAMPLE 1

A comparative surface-treated water-absorbent resin (1) was obtained inthe same way as of Example 1 except that the blender was changed into acontinuous high-speed-stirring blender equipped with two straight pipenozzles (C1′) having an internal diameter of 6 mm in stead of thehydraulic hollow cone spray nozzles made by H. Ikeuchi & Co., Ltd.

The aqueous surface-crosslinking agent solution (B1) was supplied in aform of liquid drop from the straight pipe nozzles (C1′) as used.Therefore, the spray angle and the dispersing area of a spray-dispersingstate projected onto a sectional area of the blending apparatus couldnot be measured.

After finishing the above procedure, the internal portion of the blenderwas observed. Then, the growth of piled materials was partially observedon the stirring paddle.

The resultant comparative surface-treated water-absorbent resin (1) hadparticles which were agglomerated rigidly, and could not be crashed, andweren't passed through a sieve having a mesh opening of 850 μm.Therefore, the properties of the resultant comparative surface-treatedwater-absorbent resin (1) were listed in Table 1. However, the value ofthe particle size distribution as measured includes particles notpassing through a sieve having a mesh opening of 850 μm. The absorptioncapacity without load and absorption capacity under a load were measuredby removing the particles not passing through a sieve having a meshopening of 850 μm.

COMPARATIVE EXAMPLE 2

A comparative surface-treated water-absorbent resin (2) was obtained inthe same way as of Example 1 except that the blender was changed into acontinuous high-speed-stirring blender equipped with an air-atomizingnozzle (C2′, its spray setup number was SU1, and its spray pattern was acircular and full cone shape; made by Spraying Systems Co., Japan).

The air-atomizing nozzle (C2′, its spray setup number was SU1, made bySpraying Systems Co., Japan) was used. Therefore, the spray angle of theaqueous surface-treating agent solution from the air-atomizing nozzlewas 18°, and the dispersing area of a spray-dispersing state projectedonto a sectional area perpendicular to the shaft direction of theblending apparatus accounted for about 20%.

After finishing the above procedure, the internal portion of the blenderwas observed. Then, piled materials were observed on the stirring paddleand shaft.

The resultant comparative surface-treated water-absorbent resin (2) hadparticles which were agglomerated rigidly, and could not be crashed, andweren't passed through a sieve having a mesh opening of 850 μm.Therefore, the properties of the resultant comparative surface-treatedwater-absorbent resin (2) were listed in Table 1. However, the value ofthe particle size distribution as measured includes particles notpassing through a sieve having a mesh opening of 850 μm. The absorptioncapacity without load and absorption capacity under a load were measuredby removing the particles not passing through a sieve having a meshopening of 850 μm.

EXAMPLE 4

The granulation was carried out in order to decrease the amount of thesurface-treated water-absorbent resin (1) as passed through a sievehaving a mesh opening of 150 μm. That is to say, the water-absorbentresin (1) as a water-absorbent resin (A2) before modifying was suppliedinto a continuous high-speed-stirring blender (a turbulizer made byHosokawa Micron Co., Ltd.) equipped with two hydraulic hollow cone spraynozzles (C1, 1/4M-K-040, made by H. Ikeuchi & Co., Ltd.; their spraypatterns were circular and hollow cone shapes) with a feeding speed of100 kg/hr, and water as a liquid material (B4) was blended therewith sothat the amount of the water as added would be adjusted to 5 weight %relative to the weight of the water-absorbent resin (A2). Then, theresultant mixture was left still at 80° C. for 1 hour to cure, and theentirety was passed through a sieve having a mesh opening of 850 μm,thus obtaining a modified (granulated) water-absorbent resin (4).

The hydraulic hollow cone spray nozzles (C1, 1/4M-K-040) were used.Therefore, the spray angle of the water (B4) was 70°, and the dispersingarea of a spray-dispersing state projected onto a sectional area of theblending apparatus accounted for about 89% by the nozzles.

After finishing the above procedure, the internal portion of the blenderwas observed. Then, large piled materials were observed little.

The properties of the resultant modified (granulated) water-absorbentresin (4) were listed in Table 2.

EXAMPLE 5

A modified (granulated) water-absorbent resin (5) was obtained in thesame way as of Example 4 except that the blender was changed into acontinuous high-speed-stirring blender equipped with two hydraulic flatspray nozzles (C2, 1/4M-V-115-05, made by H. Ikeuchi & Co., Ltd.; theirspray patterns were double-convex-lens and elliptic cone shapes).Incidentally, the hydraulic flat spray nozzles (C2, 1/4M-V-115-05) wereattached to the blender very carefully so that the spray angle would bethe largest when the dispersing area of a spray-dispersing state wasprojected onto a sectional area perpendicular to the stirring shaftdirection of the continuous high-speed-stirring blending apparatus.

The hydraulic flat spray nozzles (C2, 1/4M-V-115-05) were used.Therefore, the spray angle was 110°, and the dispersing area of aspray-dispersing state projected onto a sectional area of the blendingapparatus accounted for about 97% by the nozzles.

After finishing the above procedure, the internal portion of the blenderwas observed. Then, large piled materials were observed little.

The properties of the resultant modified (granulated) water-absorbentresin (5) were listed in Table 2.

EXAMPLE 6

A modified (granulated) water-absorbent resin (6) was obtained in thesame way as of Example 4 except that the blender was changed into acontinuous high-speed-stirring blender equipped with a hydraulic hollowcone spray nozzle (C3, 1/4M-K-100, made by H. Ikeuchi & Co., Ltd.; itsspray pattern was a circular and hollow cone shape (a hollow cone sprayshape)).

The hydraulic hollow cone spray nozzle (C3, 1/4M-K-100) was used.Therefore, the spray angle of the water (B4) was 70°, and the dispersingarea of a spray-dispersing state projected onto a sectional area of theblending apparatus accounted for about 77%.

After finishing the above procedure, the internal portion of the blenderwas observed. Then, large piled materials were observed little.

The properties of the resultant modified (granulated) water-absorbentresin (6) were listed in Table 2.

COMPARATIVE EXAMPLE 3

A comparative modified (granulated) water-absorbent resin (3) wasobtained in the same way as of Example 4 except that the blender waschanged into a continuous high-speed-stirring blender equipped with twohydraulic fiat spray nozzles (C3′, 1/4M-V-040-05, made by H. Ikeuchi &Co., Ltd.; their spray patterns were double-convex-lens and ellipticcone shapes). Incidentally, the hydraulic flat spray nozzles (C3′,1/4M-V-040-05) were attached to the blender very carefully so that thespray angle would be the largest when the dispersing area of aspray-dispersing state was projected onto a sectional area perpendicularto the stirring shaft direction of the continuous high-speed-stirringblending apparatus.

The spray angle was 40° by use of the hydraulic fiat spray nozzles (C3′,1/4M-V-040-05) when the dispersing area of a spray-dispersing state wasprojected onto a sectional area perpendicular to the stirring shaftdirection of the blending apparatus. The dispersing area of aspray-dispersing state projected onto a sectional area perpendicular tothe shaft direction of the blending apparatus accounted for about 67% bythe nozzles.

After finishing the above procedure, the internal portion of the blenderwas observed. Then, piled materials were observed on the stirring paddleand shaft.

The resultant comparative modified (granulated) water-absorbent resin(3) had particles which were agglomerated rigidly, and could not becrashed, and weren't passed through a sieve having a mesh opening of 850μm. Therefore, the properties of the resultant comparativewater-absorbent resin (3) were listed in Table 2. However, the value ofthe particle size distribution as measured includes particles notpassing through a sieve having a mesh opening of 850 μm.

COMPARATIVE EXAMPLE 4

A comparative modified (granulated) water-absorbent resin (4) wasobtained in the same way as of Example 4 except that the blender waschanged into a continuous high-speed-stirring blender equipped with twohydraulic flat spray nozzles (C2, 1/4M-V-115-05, made by H. Ikeuchi &Co., Ltd.; their spray patterns were double-convex-lens and ellipticcone shapes). Incidentally, the hydraulic flat spray nozzles (C2,1/4M-V-115-05) were attached to the blender very carefully so that thespray angle would be the smallest when the dispersing area of aspray-dispersing state was projected onto a sectional area perpendicularto the stirring shaft direction of the continuous high-speed-stirringblending apparatus.

When the hydraulic flat spray nozzles (C2, 1/4M-V-115-05) were used,they were attached carefully so that the spray angle would be thesmallest in the above way. Therefore, the spray angle was 10° when thedispersing area of a spray-dispersing state was projected onto asectional area perpendicular to the stirring shaft direction of theblending apparatus. The dispersing area of a spray-dispersing stateprojected onto a sectional area of the blending apparatus accounted forabout 23% by the nozzles.

After finishing the above procedure, the internal portion of the blenderwas observed. Then, piled materials were observed on the stirring paddleand shaft.

The resultant comparative modified (granulated) water-absorbent resin(4) had particles which were agglomerated rigidly, and could not becrashed, and weren't passed through a sieve having a mesh opening of 850μm. Therefore, the properties of the resultant comparativewater-absorbent resin (4) were listed in Table 2. However, the value ofthe particle size distribution as measured includes particles notpassing through a sieve having a sieve opening of 850 μm.

TABLE 1 Comparative Comparative Example 1 Example 2 Example 3 Example 1Example 2 Absorption capacity 34.3 35.1 35.3 34.9 34.7 without a load(g/g) Absorption capacity 27.7 27.5 27.0 23.2 24.9 under a load (g/g)Particle size distribution Not smaller than 850 μm 0.0 0.0 0.0 0.9 0.8850 to 500 μm 29.7 29.4 29.1 30.0 29.1 500 to 300 μm 45.0 44.7 45.7 43.544.5 300 to 150 μm 19.3 19.7 18.9 18.5 18.2 Smaller than 150 μm 6.0 6.26.3 7.1 7.4 The absorption capacity under a load was measured by use ofa physiological saline solution as a swollen liquid.

TABLE 2 Comparative Comparative Particle size distribution Example 4Example 5 Example 6 Example 3 Example 4 Not smaller than 850 μm 0.0 0.00.0 1.2 1.8 850 to 500 μm 33.3 32.9 32.2 32.0 31.5 500 to 300 μm 46.446.1 45.5 45.3 45.5 300 to 150 μm 16.4 17.7 17.8 16.3 15.7 Smaller than150 μm 3.9 3.3 4.5 5.2 5.5

From Table 1, the resultant surface-crosslinked water-absorbent resinobtained by blending according to the present invention processdisplayed a higher absorption capacity under a load, and was notobserved to produce very hard agglomerated materials such that could notbe passed through a sieve having a mesh opening of 850 μm in comparisonwith those obtained by a blending process using the straight pipenozzles. In addition, it would be understood that: the growth of piledmaterials due to adding the liquid material excessively was not observedin the blending apparatus, and the water-absorbent resin was uniformlyblended with the aqueous surface-crosslinking agent solution as theliquid material.

From Table 2, the ratio of particles smaller than 150 μm was decreased,and the production of very hard agglomerated materials such that couldnot be passed through a sieve having a mesh opening of 850 μm was notobserved, because the blending process according to the presentinvention was applied. It would be understood that the present processwas effective for blending much aqueous solution for the purpose ofdecreasing dusts of water-absorbent resins as caused.

From the above results, the blending process according to the presentinvention can be regarded as a very effective process forwater-absorbent resins easily agglomerated by blending with a liquidmaterial when the uniform blending state is required.

REFERENTIAL EXAMPLE 1

An aqueous monomer solution was prepared by mixing 3,683 parts by weightof aqueous sodium acrylate solution of 37 weight %, 562 parts by weightof acrylic acid, 4.26 parts by weight of polyethylene glycol diacrylate(average unit of ethylene oxide: 8), and 1,244 parts by weight ofdeionized water. In a monomer degassing vessel, nitrogen was blown into1 liter of this aqueous monomer solution with a feeding rate of 0.8liter/minute for 30 minutes in order to remove the dissolved oxygen inthe aqueous solution. Next, 4.5 parts by weight of an aqueous sodiumperfulfate solution of 5 weight %, 4.0 part by weight of an aqueousL-ascorbic acid solution of 0.5 weight %, and 4.4 parts by weight of2,2′-azobis-(2-amidinopropane)dihydrochloride solution were mixed withthe aqueous monomer solution from a polymerization initiator vesselrespectively. While 3.2 parts by weight of an aqueous hydrogen peroxidesolution of 3.5 weight % was supplied, the aqueous monomer solutionmixed with the polymerization initiator was supplied onto a belt tocarry out a stationary polymerization continuously.

The full length of the belt was 3.5 m, and the interval from a portionfor supplying the aqueous monomer solution to 1 m toward the drivingdirection was equipped with a cooling apparatus for cooling the surfaceof the belt, and the residual portion was equipped with a heat-treatingapparatus for heating the surface of the belt. The aqueous monomersolution supplied onto the belt formed a viscous gel material afterabout one minute, and the temperature was reached to the maximum after 7minutes. The maximum temperature was 80° C. Continuously, thepolymerized gel was matured in a heating zone of 60° C., thus obtaininga transparent hydrogel. This hydrogel was crushed with a meat chopper,and dried in a hot-blow dryer for 65 minutes at 160° C. The resultantdried product was crushed, thus obtaining a water-absorbent resin (A3)having an average particle diameter of 350 μm, wherein particles havingparticle diameters of smaller than 150 μm was 5 weight % of thewater-absorbent resin. Its absorption capacity and extractable contentwere 52 times (52 g/g) and 12% respectively.

EXAMPLE 7

A mixed composition comprising 0.5 part of 1,3-propanediol, 0.5 part ofpropylene glycol, 3.0 parts of water, and 0.5 part of ethanol wasblended into 100 parts of the water-absorbent resin (A3) as obtained inReferential Example 1 with a turbulizer. The mixture as obtained washeat-treated for one hour in a paddle-type dryer, wherein the internalwall (heat medium) temperature of the paddle-type dryer was 185° C., andthe atmosphere of the space portion in the dryer was adjusted to have adew point of 40° C. and a temperature of 97° C., thus obtaining awater-absorbent resin (absorbing agent) (7). The results were listed inTable 3.

EXAMPLE 8

A water-absorbent resin (absorbing agent) (8) was obtained in the sameway as of Example 7 except that the atmosphere of the space portion inthe paddle-type dryer was adjusted to have a dew point of 50° C. and atemperature of 119° C. The results were listed in Table 3.

EXAMPLE 9

A mixed composition comprising 0.5 part of 1,3-propanediol, 0.5 part ofpropylene glycol, 3.0 parts of water, and 0.5 part of ethanol wasblended into 100 parts of the water-absorbent resin (A3) as obtained inReferential Example 1 with a turbulizer. The mixture as obtained washeat-treated for one hour in a double-arm type kneader, wherein theinternal wall (heat medium) temperature of the double-arm type kneaderwas 185° C., and the atmosphere of the space portion in the dryer wasadjusted to have a dew point of 60° C. and a temperature of 145° C.,thus obtaining a water-absorbent (absorbing agent) (9). The results werelisted in Table 3.

COMPARATIVE EXAMPLE 5

A comparative water-absorbent resin (comparative absorbing agent) (5)was obtained in the same way as of Example 7 except that the atmosphereof the space portion in the paddle-type dryer was adjusted to have a dewpoint of 25° C. and a temperature of 88° C. The results were listed inTable 3.

COMPARATIVE EXAMPLE 6

A comparative water-absorbent resin (comparative absorbing agent) (6)was obtained in the same way as of Example 7 except that the atmosphereof the space portion in the paddle-type dryer was adjusted to have a dewpoint of 100° C. and a temperature of 142° C. The results were listed inTable 3.

TABLE 3 Atmosphere Dew point Temperature Absorption Absorption capacity(° C.) (° C.) capacity (g/g) under a load (g/g) Example 7Water-absorbent resin 40  97 35 32 (absorbing agent) (7) Example 8Water-absorbent resin 50 119 33 35 (absorbing agent) (8) Example 9Water-absorbent resin 60 145 35 30 (absorbing agent) (9) ComparativeComparative 25  88 40 14 Example 5 water-absorbent resin (comparativeabsorbing agent) (5) Comparative Comparative 100  142 37 13 Example 6water-absorbent resin (comparative absorbing agent) (6) The absorptioncapacity under a load was measured by use of synthetic urine as aswollen liquid.

EXAMPLE 10

In a kneader equipped with two sigma type blades, an aqueous acrylicacid salt monomer solution having a monomer concentration of 38 weight %and a neutralization ratio of 75 mol % was prepared, wherein the aqueousmonomer solution comprised an aqueous sodium acrylate solution, acrylicacid and water. Polyethylene glycol diacrylate (average ethylene oxideunit: 8) as an internal-crosslinking agent was dissolved therein so thatits concentration would be adjusted to 0.035 mol % of the monomer. Next,the amount of dissolved oxygen of the aqueous monomer solution wasdecreased and the entirety of the reaction apparatus was replaced withnitrogen gas by introducing the nitrogen gas into the aqueous solution.Next, while the two sigma blades were rotated, 0.05 mol % of sodiumpersulfate and 0.0003 mol % of L-ascorbic acid were added as apolymerization initiator to carry out a stirring polymerization in thekneader, thus obtaining a hydrogel polymer having an average particlediameter of about 2 mm after 40 minutes.

The resultant hydrogel polymer was dried in a hot air dryer adjusted ata temperature of 170° C. for 60 minutes. The resultant dried product waspulverized with a roller mill, and then classified with a mesh of 850 μmto remove particles larger than 850 μm, thus obtaining a water-absorbentresin (A4). The resultant water-absorbent resin (A4) had a particle sizedistribution such that the average particle diameter was 350 μm and theratio of particles having particle diameters of smaller than 150 μm was7 weight %, and exhibited an absorption capacity of 45 times (45 g/g).

The above water-absorbent resin (A4) was supplied into a continuoushigh-speed-stirring blender (a turbulizer made by Hosokawa Micron Co.,Ltd.) equipped with a hydraulic hollow cone spray nozzle (C1,1/4M-K-040, made by H. Ikeuchi & Co., Ltd.; its spray pattern was acircular and hollow cone shape) with a feeding speed of 100 kg/hr, andan aqueous surface-crosslinking agent solution prepared with a blendingratio of 1,4-butandiol: propylene glycol: water=1:1:6 as a liquidmaterial (B10) was blended therewith so that the amount of the aqueoussolution as added would be adjusted to 4 weight % relative to the weightof the water-absorbent resin (A4). After the resultant mixture washeat-treated for 50 minutes in a paddle-type dryer of 190° C.(water-absorbent resin temperature (material temperature)), wherein theatmosphere of the upper space inside of the paddle-type dryer had a dewpoint of 50° C. and a temperature of 160° C., the entirety was passedthrough a sieve having a mesh opening of 850 μm, thus obtaining asurface-treated water-absorbent resin (10).

The hydraulic hollow cone spray nozzle (C1, 1/4M-K-040; its spraypattern was a circular and hollow cone shape) was used. Therefore, thespray angle of the aqueous surface-treating agent solution from thehydraulic hollow cone spray nozzle was 70°, and the dispersing area of aspray-dispersing state projected onto a sectional area perpendicular tothe stirring shaft direction of the blending apparatus accounted forabout 77%.

After finishing the above procedure, the internal portion of the blenderwas observed. Then, large piled materials were observed little.

The properties of the resultant surface-treated water-absorbent resin(10) were listed in Table 4.

EXAMPLE 11

All the procedures were carried out in the same way as of Example 10except that the aqueous surface-crosslinking agent solution was replacedwith an aqueous surface-crosslinking agent solution (B11) prepared witha blending ratio of 1,3-dioxolane-2-one: water: ethanol=1:1:1 as theliquid material, and was blended so that the amount of the aqueoussolution as added would be adjusted to 7.5 weight % relative to theweight of the water-absorbent resin (A4).

The properties of the resultant water-absorbent resin (11) were listedin Table 4.

As is shown in Table 4, when the polyhydric alcohol was not used, theproperties of Example 11 were inferior to that of Example 10.Incidentally, Example 11 is an example for the production process, butis not for the water-absorbent resin.

EXAMPLE 12

All the procedures were carried out in the same way as of Example 10except that the mixture resultant from the water-absorbent resin (A4)and the liquid material (B10) was heat-treated treated for 50 minutes ina paddle-type dryer of 190° C. (water-absorbent resin temperature(material temperature)), wherein the atmosphere of the upper spaceinside of the paddle-type dryer had a dew point of 40° C. and atemperature of 80° C.

The properties of the resultant water-absorbent resin (12) were listedin Table 4.

As is shown in Table 4, the properties of Example 12 were inferior tothat of Example 10. Incidentally, Example 12 is an example for theproduction process, but is not for the water-absorbent resin.

TABLE 4 Example 10 Example 11 Example 12 Single-layer 600 to 300 μm 10min. 31.8 29.2 32.0 absorption capacity 60 min. 37.9 37.2 39.6 under aload (g/g) 300 to 150 μm 10 min. 32.2 30.3 29.8 60 min. 34.3 32.3 33.9Time variation of single-layer 600 to 300 μm 0.84 0.78 0.81 absorptioncapacity under a load 300 to 150 μm 0.94 0.94 0.88 Variation betweenparticles of 10 min. 1.01 1.03 0.93 single-layer absorption capacity 60min. 0.91 0.87 0.87 under a load Index of uniform surface-treatment 0.730.66 0.58 Absorption capacity (g/g) 34 32 34

Various details of the invention may be changed without departing fromits spirit not its scope. Furthermore, the foregoing description of thepreferred embodiments according to the present invention is provided forthe purpose of illustration only, and not for the purpose of limitingthe invention as defined by the appended claims and their equivalents.

What is claimed is:
 1. A production process for a water-absorbent resin,comprising the steps of: blending a liquid material and awater-absorbent resin; and heating the resultant mixture to produce amodified water-absorbent resin, with the production process comprisingthe step of spray-blending a water-absorbent resin (A) and a liquidmaterial (B) with a blending apparatus equipped with a spray nozzle (C),and spraying the liquid material (B) from the spray nozzle (C) in acircular and hollow cone shaped spray pattern.
 2. A production processfor a water-absorbent resin, comprising the steps of: blending a liquidmaterial and a water-absorbent resin; and heating the resultant mixtureto produce a modified water-absorbent resin, with the production processcomprising the step of spray-blending a water-absorbent resin (A) and aliquid material (B) with a blending apparatus equipped with a spraynozzle (C), and spraying the liquid material (B) from the spray nozzle(C) in a double-convex-lens and elliptical shaped spray pattern, whereinsaid spray nozzle (C) is attached to said blending apparatus to form aspray angle of not less than 50° onto an area on said water-absorbentresin, when said dispersing area of a spray-dispersing state isprojected onto a sectional area perpendicular to a transfer direction ofsaid water absorbent resin.
 3. A production process for awater-absorbent resin, comprising the steps of: blending a liquidmaterial (B) and a water-absorbent resin (A); and heating the resultantmixture to produce a modified water-absorbent resin, with the productionprocess comprising the step of heat-treating said mixture ofwater-absorbent resins (A) and liquid material (B) under an atmospherehaving a dew point of not higher 60° C. and a temperature of not lowerthan 90° C., wherein the modified water-absorbent resin is obtainedafter a drying step following a pulverization step.
 4. A productionprocess for a water-absorbent resin, comprising the steps of: blending aliquid material and a water-absorbent resin; and heating the resultantmixture to produce a modified water-absorbent resin, with the productionprocess comprising the steps of: spray-blending a water-absorbent resin(A) and a liquid material (B) with a blending apparatus equipped with aspray nozzle (C); and heat-treating, wherein the liquid material (B) issprayed from the spray nozzle (C) in a circular and hollow cone shapedspray pattern in the spray-blending step, and wherein the heat-treatingstep is carried out under an atmosphere having a dew point of not higherthan 60° C. and a temperature of not lower than 90° C.
 5. A productionprocess for a water-absorbent resin, comprising the steps of: blending aliquid material and a water-absorbent resin; and heating the resultantmixture in order to produce a modified water-absorbent resin, with theproduction process comprising the steps of: spray-blending awater-absorbent resin (A) and a liquid material (B) with a blendingapparatus equipped with a spray nozzle (C); and heat-treating, whereinthe liquid material (B) is sprayed from the spray nozzle (C) in adouble-convex-lens and elliptical cone shaped spray pattern in thespray-blending step, and wherein the heat-treating step is carried outunder an atmosphere having a dew point of not higher than 60° C. and atemperature of not lower than 90° C., and wherein said spray nozzle (C)is attached to said blending apparatus to form a spray angle of not lessthan 50° onto an area on said water-absorbent resin, when saiddispersing area of a spray-dispersing state is projected onto asectional area perpendicular to a transfer direction of said waterabsorbent resin.
 6. A production process for a water-absorbent resinaccording to claim 1, wherein the maximum spray angle of the liquidmaterial (B) from the spray nozzle (C) is not less than 50° C.
 7. Aproduction process for a water-absorbent resin according to claim 1,wherein the blending apparatus equipped with the spray nozzle (C) is acontinuous blending apparatus comprising an agitation shaft having aplurality of paddles.
 8. A production process for a water-absorbentresin according to claim 7, wherein the area of a spray-dispersing stateof the liquid material (B) projected onto a sectional area which isperpendicular to the axis direction of the blending apparatus andincludes a spraying point of the spray nozzle (C) accounts for not lessthan 70% of the sectional area perpendicular to the axis direction ofthe blending apparatus.
 9. A production process for a water-absorbentresin according to claim 1, wherein the blending apparatus is equippedwith the plurality of spray nozzles (C).
 10. A production process for awater-absorbent resin according to claim 1, wherein the liquid material(B) is an aqueous solution of a surface-crosslinking agent which forms acovalent bond by reacting with a functional group of the water-absorbentresin (A), and which further comprises the step of heat-treating themixture resultant from the blending step at a water-absorbent resintemperature of 80 to 250° C.
 11. A production process for awater-absorbent resin according to claim 10, wherein the liquid material(B) is an aqueous solution including at least one material selected fromthe group consisting of polyhydric alcohols, polyglycidyl compounds,1,3-dioxolan-2-on, poly(2-oxazolidinone), bis(2-oxazolidinone), andmono(2-oxazolidinone).
 12. A production process for a water-absorbentresin according to claim 11, wherein the liquid material (B) is anaqueous surface-crosslinking agent solution including a polyhydricalcohol.
 13. A production process for a water-absorbent resin accordingto claim 2, wherein the blending apparatus equipped with the spraynozzle (C) is a continuous blending apparatus comprising an agitationshaft having a plurality of paddles.
 14. A production process for awater-absorbent resin according to claim 13, wherein the area of aspray-dispersing state of the liquid material (B) projected onto asectional area which is perpendicular to the axis direction of theblending apparatus and includes a spraying point of the spray nozzle (C)accounts for not less than 70% of the sectional area perpendicular tothe axis direction of the blending apparatus.
 15. A production processfor a water-absorbent resin according to claim 2, wherein the blendingapparatus is equipped with the plurality of spray nozzles (C).
 16. Aproduction process for a water-absorbent resin according to claim 2,wherein the liquid material (B) is an aqueous solution of asurface-crosslinking agent which forms a covalent bond by reacting witha functional group of the water-absorbent resin (A), and which furthercomprises the step of heat-treating the mixture resultant from theblending step at a water-absorbent resin temperature of 80 to 250° C.17. A production process for a water-absorbent resin according to claim16, wherein the liquid material (B) is an aqueous solution including atleast one material selected from the group consisting of polyhydricalcohols, polyglycidyl compounds, 1,3-dioxolan-2-on,poly(2-oxazolidinone), bis(2-oxazolidinone), and mono(2-oxazolidinone).18. A production process for a water-absorbent resin according to claim17, the liquid material (B) is an aqueous surface-crosslinking agentsolution including a polyhydric alcohol.
 19. A production process for awater-absorbent resin according to claim 3, wherein the liquid material(B) is spray-blended with a blending apparatus equipped with a spraynozzle (C).
 20. A production process for a water-absorbent resinaccording to claim 19, wherein the maximum spray angle of the liquidmaterial (B) from the spray nozzle (C) is not less than 50°.
 21. Aproduction process for a water-absorbent resin according to claim 19,the blending apparatus equipped with the spray nozzle (C) is acontinuous blending apparatus comprising an agitation shaft having aplurality of paddles.
 22. A production process for a water-absorbentresin according to claim 21, wherein the area of a spray-dispersingstate of the liquid material (B) projected onto a sectional area whichis perpendicular to the axis direction of the blending apparatus andincludes a spraying point of the spray nozzle (C) accounts for not lessthan 70% of the sectional area perpendicular to the axis direction ofthe blending apparatus.
 23. A production process for a water-absorbentresin according to claim 19, wherein the blending apparatus is equippedwith the plurality of spray nozzles (C).
 24. A production process for awater-absorbent resin according to claim 3, wherein the liquid material(B) is an aqueous solution of a surface-crosslinking agent which forms acovalent bond by reacting with a functional group of the water-absorbentresin (A), and which further comprises the step of heat-treating themixture resultant from the blending step at a water-absorbent resintemperature of 80 to 250° C.
 25. A production process for awater-absorbent resin according to claim 24, wherein the liquid material(B) is an aqueous solution including at least one material selected fromthe group consisting of polyhydric alcohols, polyglycidyl compounds,1,3-dioxolan-2-on, poly(2-oxazolidinone), bis(2-oxazolidinone), andmono(2-oxazolidinone).
 26. A production process for a water-absorbentresin according to claim 25, the liquid material (B) is an aqueoussurface-crosslinking agent solution including a polyhydric alcohol. 27.A production process for a water-absorbent resin according to claim 4,wherein the maximum spray angle of the liquid material (B) from thespray nozzle (C) is not less than 50°.
 28. A production process for awater-absorbent resin according to claim 4, wherein the blendingapparatus equipped with the spray nozzle (C) is a continuous blendingapparatus comprising an agitation shaft having a plurality of paddles.29. A production process for a water-absorbent resin according to claim28, the area of a spray-dispersing state of the liquid material (B)projected onto a sectional area which is perpendicular to the axisdirection of the blending apparatus and includes a spraying point of thespray nozzle (C) accounts for not less than 70% of the sectional areaperpendicular to the axis direction of the blending apparatus.
 30. Aproduction process for a water-absorbent resin according to claim 4,wherein the blending apparatus is equipped with the plurality of spraynozzles (C).
 31. A production process for a water-absorbent resinaccording to claim 4, wherein the liquid material (B) is an aqueoussolution of a surface-crosslinking agent which forms a covalent bond byreacting with a functional group of the water-absorbent resin (A), andwhich further comprises the step of heat-treating the mixture resultantfrom the blending step at a water-absorbent resin temperature of 80 to250° C.
 32. A production process for a water-absorbent resin accordingto claim 21, wherein the liquid material (B) is an aqueous solutionincluding at least one material selected from the group consisting ofpolyhydric alcohols, polyglycidyl compounds, 1,3-dioxolan-2-on,poly(2-oxazolidinone), bis(2-oxazolidinone), and mono(2-oxazolidinone).33. A production process for a water-absorbent resin according to claim32, the liquid material (B) is an aqueous surface-crosslinking agentsolution including a polyhydric alcohol.
 34. A production process for awater-absorbent resin according to claim 5, wherein the blendingapparatus equipped with the spray nozzle (C) is a continuous blendingapparatus comprising an agitation shaft having a plurality of paddles.35. A production process for a water-absorbent resin according to claim34, wherein the area of a spray-dispersing state of the liquid material(B) projected onto a sectional area which is perpendicular to the axisdirection of the blending apparatus and includes a spraying point of thespray nozzle (C) accounts for not less than 70% of the sectional areaperpendicular to the axis direction of the blending apparatus.
 36. Aproduction process for a water-absorbent resin according to claim 5,wherein the blending apparatus is equipped with the plurality of spraynozzles (C).
 37. A production process for a water-absorbent resinaccording to claim 5, wherein the liquid material (B) is an aqueoussolution of a surface-crosslinking agent which forms a covalent bond byreacting with a functional group of the water-absorbent resin (A), andwhich further comprises the step of heat-treating the mixture resultantfrom the blending step at a water-absorbent resin temperature of 80 to250° C.
 38. A production process for a water-absorbent resin accordingto claim 37, wherein the liquid material (B) is an aqueous solutionincluding at least one material selected from the group consisting ofpolyhydric alcohols, polyglycidyl compounds, 1,3-dioxolan-2-on,poly(2-oxazolidinone), bis(2-oxazolidinone), and mono(2-oxazolidinone).39. A production process for a water-absorbent resin according to claim38, the liquid material (B) is an aqueous surface-crosslinking agentsolution including a polyhydric alcohol.